The vacancy on the court, which is now evenly split 4-4 between its conservative and liberal wings, had Republicans calling on President Barack Obama to refrain from choosing a successor to the right-leaning Scalia while Democrats urged Obama to do as the U.S. Constitution requires and put forward a candidate to face confirmation in an albeit hostile Senate.

The prospect of such a battle drew swift and furious comment from candidates vying to be elected president in November.

Facing off in a debate only hours after the 79-year-old Scalia's death was announced, some Republican presidential candidates seized the moment to caution voters that their party's front-runner, billionaire businessman Donald Trump, could not be trusted to nominate a stalwart conservative.

"If Donald Trump is president, he will appoint liberals," charged U.S. Senator Ted Cruz of Texas during the debate in South Carolina, which holds a Republican nominating contest next Saturday.

"Two branches of government hang in the balance, not just the presidency, but the Supreme Court," Cruz said. "If we get this wrong, if we nominate the wrong candidate, the Second Amendment, life, marriage, religious liberty, every one of those hangs in the balance."

Senator Lindsey Graham of South Carolina also took a shot at Trump. "Donald Trump is not a conservative, so I don’t trust him to pick a judge," Graham said before the debate. A real estate mogul, Trump has supported Democratic politicians in the past.

Trump, who also has taken several positions at odds with Republican orthodoxy, joined other candidates at the debate in insisting that Senator Mitch McConnell, Republican majority leader in the Senate, stand by his promise to block any Obama high court choice.

“It’s up to Mitch McConnell and everyone else to stop it," Trump, a former reality TV show host, said. "It’s called delay, delay, delay."

Under the U.S. system, the president nominates justices for the nine-member court and the Senate confirms them. The last justice to be approved by the Senate of the opposite party during an election year was Justice Anthony Kennedy in 1988.

Obama has already indicated that he intends to send a choice to the Senate in coming weeks, meaning that the nominee will be heavily scrutinized by presidential candidates in both parties - and more than likely be opposed by the majority of Republicans.

"The court may genuinely be a major issue this year," said David Axelrod, a former top political adviser to Obama. "It will be a hell of a fight."

SOCIAL ISSUES ON DOCKET

Criticism of the court, which in recent years has upheld Obama's sweeping healthcare plan and legalized same-sex marriage, has already been a thread running through several Republican candidates' campaigns.

The conservative majority on the court had appeared poised to invalidate Obama's immigration and climate-change policies. The loss of Scalia, considered to be a lodestar of conservative legal thought, and the potential swing of the court to the left, ensures that whatever drama plays out in the Senate this year will be mirrored on the campaign trail.

“There is no more clarifying debate in politics these days than when it comes to Supreme Court nominees,” said Jim Manley, a Democratic strategist and former aide to Senator Harry Reid of Nevada. "This now is for all the marbles."

Reid was majority leader of the Senate when it confirmed previous Obama court nominees Sonia Sotomayor and Elena Kagan. Manley called McConnell's threat not to allow a vote on a potential Scalia replacement "completely beyond the pale."

Democratic candidate Hillary Clinton seemed inclined to make McConnell's threat a campaign issue.

"The Republicans in the Senate and on the campaign trail who are calling for Justice Scalia’s seat to remain vacant dishonor our Constitution," Clinton said in a statement.

Axelrod said that the issue could help Clinton, locked in a tight race with U.S. Senator Bernie Sanders of Vermont.

“I think it will make electability and experience in this realm more important," he said.

OBAMA'S OPTIONS

Rick Hasen, a law professor at the University of California, Irvine, and a longtime observer of the Supreme Court nomination process, said Obama likely has two options.

"He could nominate a more liberal candidate who would have no real chance of getting through a Republican Senate - in which case this would become a salient political issue in 2016," Hasen said. Or he could nominate a more moderate candidate who might gain enough Republican support to gain approval, he said.

There are risks to both approaches: A Republican obstruction of a liberal nominee would animate the Democratic Party's progressive base in an election year but would leave the court without a potentially tie-breaking vote for perhaps a year.

That same Democratic base might view a moderate nominee as a betrayal, while conservative Republican voters likely would frown on any senator who voted to approve an Obama choice.

Manley said that McConnell has already shown that he is unwilling to support any choice made by Obama and that the White House must act aggressively. “The president should go forward and nominate the most liberal candidate possible,” he said.

Given the need to fire up its most passionate voters, that might just be exactly what Republican candidates want as well.

(This version of the story was refiled to fix the mistyped word "then" to "than" in paragraph 16.)

(Writing by James Oliphant; Reporting by Ginger Gibson and James Oliphant; Additional reporting by Steve Holland and Jeff Mason; Editing by Howard Goller)

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