Republican lawmakers and presidential candidates hardened their positions today on blocking a move by US President Barack Obama to fill the seat left by the late conservative Justice Antonin Scalia on the Supreme Court.

The lifetime appointment would help decide some of the most divisive issues facing Americans.

The next justice could tilt the balance of the nation's highest court, which was left with four conservatives and four liberals. The vacancy has quickly become an issue in the 2016 presidential race.

"We ought to make the 2016 election a referendum on the Supreme Court," US Senator and Republican presidential candidate Ted Cruz said.

The normally nine-justice court is set to decide this year its first major abortion case in nearly a decade, as well as cases on voting rights, affirmative action and immigration.

Mr Scalia, 79, died yesterday at a West Texas resort. The cause of death was a heart attack, Texas television station WFAA-TV reported today, citing a Texas county judge.

Mr Obama, a Democrat, said yesterday that he would nominate someone to fill the empty seat, setting up a battle with the Republican-controlled Senate, which must approve any nominee.

Republicans quickly vowed not to act on the vacancy until Mr Obama's successor takes office next January. Senate Democratic leader Harry Reid said failure to act would be a "shameful abdication" of the Senate's constitutional duty.

Both sides claimed history was on their side.

Mr Reid said it would be unprecedented to have a vacancy on the court for a year. In the modern era, the longest Supreme Court vacancy was 363 days after Abe Fortas resigned in May 1969.

Republicans cited 80 years of tradition in which no Supreme Court nominees were approved in presidential election years. In fact, Justice Anthony Kennedy was approved in 1988, after a bruising battle in which the Senate rejected President Ronald Reagan's first nominee, conservative Robert Bork.

Supreme Court nominations are rare, so neither side has much data to rely on in determining precedents. History is also an unreliable guide as the nomination process has become significantly more politicised in recent years.