Updated at 11:55 a.m. ET: In a dramatic victory for President Barack Obama, the Supreme Court upheld the 2010 health care law Thursday, preserving Obama’s landmark legislative achievement.

The majority opinion was written by Chief Justice John Roberts, who held that the law was a valid exercise of Congress’s power to tax.

Roberts re-framed the debate over health care as a debate over increasing taxes. Congress, he said, is “increasing taxes” on those who choose to go uninsured.

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The 2010 law, the Affordable Care Act, requires non-exempted individuals to maintain a minimum level of health insurance or pay a tax penalty.

The essence of Roberts’s ruling was:

•       “The Affordable Care Act is constitutional in part and unconstitutional in part,” Roberts wrote.

•       “The individual mandate cannot be upheld as an exercise of Congress’s power under the Commerce Clause. That Clause authorizes Congress to regulate interstate commerce, not to order individuals to engage in it.”

•       But “it is reasonable to construe what Congress has done as increasing taxes on those who have a certain amount of income, but (who) choose to go without health insurance. Such legislation is within Congress’s power to tax.”

Roberts made a point of noting that he and the other justices “possess neither the expertise nor the prerogative to make policy judgments. Those decisions are entrusted to our Nation’s elected leaders, who can be thrown out of office if the people disagree with them. It is not our job to protect the people from the consequences of their political choices.”

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The law, Roberts wrote, “makes going without insurance just another thing the Government taxes, like buying gasoline or earning income. And if the mandate is in effect just a tax hike on certain taxpayers who do not have health insurance, it may be within Congress’s constitutional power to tax.”

Jason Reed / Reuters

A sharply divided Supreme Court on Thursday upheld the centerpiece of Obama's signature healthcare overhaul law that requires that most Americans get insurance by 2014 or pay a financial penalty.

He said “The question is not whether that is the most natural interpretation of the mandate, but only whether it is a ‘fairly possible’ one.”

He said the Supreme Court precedent is that “every reasonable construction” of a law passed by Congress “must be resorted to, in order to save a statute from unconstitutionality.” 

Dems cheer high court as galvanized GOP vows 'full repeal'

Veteran Supreme Court lawyer Tom Goldstein told NBC’s Pete Williams that “the Affordable Care Act was saved by Chief Justice John Roberts.”

Claire McAndrew of Washington, left, and Donny Kirsch of Washington celebrate outside the Supreme Court in Washington, D.C., Thursday, after a the court's ruling on health care.

Goldstein said the Obama administration “got the one vote they really needed in Chief Justice John Roberts.”

When he served in the Senate in 2005, Obama voted against confirming Roberts as chief justice, arguing that he lacked empathy for underdogs and “he has far more often used his formidable skills on behalf of the strong in opposition to the weak.”

(Twenty-one other Democratic senators, including Joe Biden, also voted against confirming Roberts. Twenty-two Democratic senators voted to confirm him.)

Obama hailed his victory: “The highest court in the land has now spoken. We will continue to implement this law and we'll work together to improve on it where we can.”

But he urged Americans to refrain from re-fighting "the political battles of two years ago" or trying to "go back to the way things were.”

The four justices joining Roberts in upholding the law were Justices Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor and Elena Kagan.

The dissenting justices were Justices Antonin Scalia, Anthony Kennedy, Clarence Thomas and Samuel Alito.

For individuals who choose to not comply with the individual insurance mandate, Congress deliberately chose to make the penalty fairly weak: only $95 for 2014; $325 for 2015; and $695 in 2016.

After 2016, that $695 amount is indexed to the consumer price index.

Congress specifically did not allow the use of liens and seizures of property as methods of enforcing the penalty.

Non-compliance with the mandate is also not subject to criminal or civil penalties under the Tax Code and interest does not accrue for failure to pay the penalty in a timely manner, according to the congressional Joint Committee on Taxation.

Chief Justice Roberts announces the Supreme Court's opinion in health care law

NBC's Pete Williams reported that Roberts reasoned that “there’s no real compulsion here” since those who do not pay the penalty for not having insurance can’t be sent to jail. “This is one of the scenarios that administration officials had considered that if the court did this they would consider it a big victory,” Williams said.

In his reaction to the court’s decision, Republican presidential contender Mitt Romney said, “What the court did today was say that Obamacare does not violate the Constitution. What they did not do was say that Obamacare is good law or that it's good policy.”

He said the ruling had made it clear “If we want to get rid of Obamacare, we're going to have replace President Obama.”

But in a major victory for the states who challenged the law, the court said that the Obama administration cannot coerce states to go along with the Medicaid insurance program for low-income people.

The financial pressure which the federal government puts on the states in the expansion of Medicaid “is a gun to the head,” Roberts wrote.

“A State that opts out of the Affordable Care Act’s expansion in health care coverage thus stands to lose not merely ‘a relatively small percentage’ of its existing Medicaid funding, but all of it,” Roberts said.

Congress cannot “penalize States that choose not to participate in that new program by taking away their existing Medicaid funding,” Roberts said.

The Medicaid provision is projected to add nearly 30 million more people to the insurance program for low-income Americans -- but the court’s decision left states free to opt out of the expansion if they choose.

Updated at 4:50 pm ET In a split decision, the Supreme Court on Monday upheld one part of a tough Arizona immigration law, but struck down other sections.

The part of the law the justices upheld requires police officers stopping someone to make efforts to verify the person’s immigration status with the federal government.

Text of the decision (PDF)

The justices struck down three other parts of the law:

• One making it a crime for an illegal immigrant to work or to seek work in Arizona;
• One which authorized state and local officers to arrest people without a warrant if the officers have probable cause to believe a person is an illegal immigrant;
• And one that made it a state requirement for immigrants to register with the federal government.

"Arizona may have understandable frustrations with the problems caused by illegal immigration" while the federal government tries to enforce immigration law, but the state "may not pursue policies that undermine federal law," wrote Justice Anthony Kennedy in the majority opinion.

The U.S Supreme Court annoucines decisions on June 25.

The court said that there are several ways for state officials to cooperate with the federal government on immigration enforcement, such as by responding to federal requests for information about when a particular non-citizen will be released from state custody.

"But the unilateral state action" to arrest and detain suspected illegal immigrants "goes far beyond these measures, defeating any need for real cooperation" between the state and the federal government.

Monday's decision is only a prelude to further litgation over what now remains of the Arionza statute.

Yuri Gripas / Reuters

Members of the media gather June 25 for a stakeout in front of U.S. Supreme Court in Washington.

NBC’s Pete Williams reported that “there are other lawsuits against this law. There are several civil liberties groups suing in Arizona, claiming that this law is racial profiling, and those cases have yet to work their way through the courts.”

Monday's decision was a partial victory for President Obama who had criticized the Arizona law, saying it “threatened to undermine basic notions of fairness that we cherish as Americans.” 

Republican presidential hopeful Mitt Romney said Monday, "I would have preferred to see the Supreme Court give more latitude to the states not less. And there are states now under this decision have less authority, less latitude to enforce immigration laws."

He added that immigration policy has "become a muddle. But it didn’t have to be this way. The president promised in his campaign that in his first year he would take on immigration ... put in place a long term program to care for those who want to come here legally, to deal with illegal immigration, to deal with securing our borders. All these things he was going to in his first year he had a Democrat house and a Democrat senate but he didn’t do it. And because he didn’t act, states and localities have tried to act and now the courts trying to get into it and sort things out and it’s a muddle"

Attorney General Eric Holder said in a statement that "I remain concerned about the impact" of the part of the law that requires police to attempt  to check the immigration status of people they stop when they have reason to suspect that the person is in the United States unlawfully.

Holder said the Justice Department "will continue to vigorously enforce federal prohibitions against racial and ethnic discrimination.  We will closely monitor the impact of S.B. 1070 to ensure compliance with federal immigration law and with applicable civil rights laws, including ensuring that law enforcement agencies and others do not implement the law in a manner that has the purpose or effect of discriminating against the Latino or any other community."

House Judiciary Committee Chairman Lamar Smith, R-Texas, said flatly that Monday's ruling was a defeat for those who seek stricter enforcement of immigration laws.

Smith said the decision "limits the ability of states to protect their citizens and communities from illegal immigrants. It is the federal government’s job to enforce our immigration laws, but President Obama has willfully neglected this responsibility. This dereliction of duty has left states to address the crime, job loss, and other costs of illegal immigration."

He added, “Unfortunately, under this Administration, today’s ruling essentially puts an end to immigration enforcement since the states no longer can step in and fill the void created by the Obama administration."

The Justice Department had moved quickly in 2010 to block enforcement of the law. The administration had argued that the Constitution vests exclusive authority over immigration matters with the federal government, not the states, and that where the federal government has pre-empted state action, no state can intrude on that federal turf.

The majority on Monday essentially agreed with that argument.

In the oral argument before the high court on April 25, Solicitor General Donald Verrilli said Arizona did not have the power to exclude or remove a person who is in the state illegally.

Although some critics of the law have contended that it would inevitably lead to targeting of Latinos simply because of appearance, speaking Spanish, or having a Spanish accent, Verrilli told the justices on April 25 “We're not making any allegation about racial or ethnic profiling in the case.”

Since enforcement of the law had been blocked by a federal judge soon after its enactment, the Obama administration did not present a record to the Supreme Court of the law leading to incidents of ethnic profiling of Latinos in the state.

Joining Kennedy's majority opinion were Chief Justice John Roberts and Justices Ruth Bader Ginsburg, Stephen Breyer and Sonia Sotomayor.

Justices Antonin Scalia, Clarence Thomas and Samuel Alito concurred in part and dissented in part.

Justice Elena Kagan, who served as Obama’s solicitor general, had recused herself from the Arizona case.

The high court’s decision comes just days after Obama announced a new administration policy of not deporting illegal immigrants under age 30 who came to the United States, or were brought to the United States before reaching age 16, who are in school, or have graduated from high school, gotten a general education certificate, or are military veterans. The illegal immigrants covered by the new administration policy will be permitted to apply for authorization to work legally in the United States.