Arkansas' State Senate on Tuesday rebuffed the governor's veto of a controversial bill that would make abortions illegal after 12 weeks of pregnancy, voting to override his decision on what would be the most restrictive ban on the procedure nationwide.

The Senate voted 20-14, just one day after Gov. Mike Beebe vetoed SB 134, also known as the "fetal heartbeat bill," NBC affiliate KARK reported.

Beebe has called the legislation "blatantly" unconstitutional and said it would "impose a ban on a woman's right to choose an elective, nontherapeutic abortion well before viability," the Arkansas Democrat-Gazette reported.

The Arkansas House must still vote to complete the override, the newspaper reported.

The state already has one of the most restrictive abortion laws in the U.S. after the Republican-led Legislature last week overrode Beebe's veto of a similar bill that set the legal abortion threshold at 20 weeks' gestation — two to four weeks earlier than most states.

That law took effect immediately but, if the House also overrides the governor's veto, the new measure wouldn't take effect until 90 days after the Legislature adjourns later this month or in early April.

The American Civil Liberties Union of Arkansas has already said it would sue if the 12-week law goes into effect.

Related: 40 years after Roe v. Wade, more states restricting abortion