President Donald Trump said Tuesday he will announce his pick on Saturday for the crucial Supreme Court seat left open by the death of justice Ruth Bader Ginsburg -- and his Republican Party vowed a quick vote to confirm the nominee.

Democratic opponents, led by presidential candidate Joe Biden, have demanded that Republicans back off on replacing Ginsburg -- who died last week -- until after the November 3 election, when they'll know whether Trump is getting a second term.

Trump set to announce Supreme Court pick
The vacancy on the US Supreme Court left by the death of Ruth Bader Ginsburg has upended the race to the November 3 presidential election. Photo: AFP / Eric BARADAT

Republicans are ignoring this, giving Trump, who has already replaced two other justices, a chance to tilt the nation's highest court to the right for decades to come, whether he beats Biden or not.

Speaking at a campaign rally in Moon Township, Pennsylvania, Trump said he would reveal his nominee at 5:00 pm (2100 GMT) on Saturday at the White House.

Republican Senator Mitt Romney
Republican Senator Mitt Romney has said he would vote to proceed with considering a nominee to replace Ruth Bader Ginsburg on the Supreme Court. Photo: AFP / NICHOLAS KAMM

"We're going to pick a great woman," he said to chants of "Fill that seat!" from the crowd. "We have great support from the Republican Party."

Trump indicated he would hold off on announcing his choice until the end of the week out of respect for the memorial services for Ginsburg taking place in the capital.

He has pledged to choose a woman for the lifetime post.

Flowers outside US Supreme Court
Supporters and fans have left flowers outside the US Supreme Court in honour of late justice Ruth Bader Ginsburg. Photo: AFP / SAUL LOEB

Leaders of the Republican majority in the Senate, which is tasked with confirming court nominees, said they have enough support to hold a vote on the nomination either before the election or at worst during the "lame-duck" session between the election and the inauguration of the next president in January.

"We will certainly do that this year," Republican Senate leader Mitch McConnell said.

US Supreme Court: potential nominees
Mini-profiles of Amy Coney Barrett and Barbara Lagoa, two women at the top of US President Donald Trump's list of potential nominees to the Supreme Court. Photo: AFP / Gal ROMA

Although two Republican senators said they believed the upper chamber of Congress should not vote at all before the election, the party's 53-47 majority is still just big enough to go ahead.

One of the other key potential Republican holdouts, frequent Trump critic Mitt Romney, said Tuesday he would move ahead with the process.

White House Press Secretary Kayleigh McEnany
IMAGES AND SOUNDBITESWhite House Press Secretary Kayleigh McEnany says President Donald Trump will nominate a third justice to the Supreme Court and that he "wants to see a fair confirmation process, wants to see one that does not look like what happened to Kavanaugh. Photo: AFPTV

"If the nominee reaches the Senate floor, I intend to vote based upon their qualifications," Romney said.

Trump welcomed what he called a "very positive statement" by Romney, who was the only Republican senator to vote for impeaching the president.

"Thank you Mitt," he said.

Senate Majority Leader Mitch McConnell
IMAGES AND SOUNDBITESSenate Majority Leader Mitch McConnell says that the filling of the Supreme Court seat of late justice Ruth Bader Ginsburg by a Trump nominee is "very likely to happen". Photo: DC POOL

Democrats argue that any Senate vote should be delayed until after the election has made clear who will lead the country from 2021.

They cite the example of 2016 when Democratic president Barack Obama's nominee to replace justice Antonin Scalia months before the election did not even get a hearing in the Republican-controlled Senate.

"Why should the American people trust the Republican senators to do anything they say when they are proving right now that their speeches mean nothing the moment the shoe is on the other foot," the Democratic leader in the Senate, Chuck Schumer, said.

Republicans say that with their control of the White House and the Senate right now, they have the right to fill court seats at any time they wish.

Adding to tensions, there is fear that leaving Ginsburg's seat unfilled -- reducing the court to eight justices -- raises the possibility of a 4-4 tie in the event of rulings related to election disputes.

Trump's Saturday announcement will set the clock ticking on what is likely to be a contentious fight in Congress as Republicans push to get the nominee confirmed at an unusually -- though not unheard of -- quick pace.

White House spokeswoman Kayleigh McEnany told reporters "we certainly believe we can" get the entire approval process done ahead of the election.

Although Democrats have no way of stopping the procedure, they will seek to inflict political pain on the Republicans over what Biden called an "abuse of power."

Schumer accused McConnell of having "defiled the Senate like no one in this generation" and said he "may very well destroy it."

As the battle lines were drawn over the court, Cindy McCain, the widow of the late senator John McCain, a Republican but frequent Trump opponent, threw her support behind Biden.

"My husband John lived by a code: country first," she said in a statement. "We are Republicans, yes, but Americans foremost.

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