Donald Trump's move to replace the late Supreme Court Justice Ruth Bader Ginsburg before the presidential election is an "abuse of power", his Democratic rival Joe Biden says.
Trump has said he will next week nominate a woman to replace the longstanding liberal justice.
Biden has urged Senate Republicans to delay a confirmation vote.
Ginsburg, a liberal icon and feminist standard-bearer, died on Friday aged 87.
Democrats fear Republicans will vote to lock in a decades-long conservative majority on the country's highest court.
The ideological balance of the nine-member court is crucial to its rulings on the most important issues in US law.
What has Biden said about Trump's decision?
During a speech at the Constitutional Center in Philadelphia on Sunday, Biden said the president had "made clear this is about power, pure and simple".
"The United States constitution allows Americans the chance to be heard - and their voice should be heard ... they should make it clear, they will not stand for this abuse of power," he said.
"I appeal to those Senate Republicans - please follow your conscience, let the people speak, cool the flames that have been engulfing our country," he said.
"Don't vote to confirm anyone nominated under the circumstances President Trump and Senator McConnell created. Don't go there."
Two Republican senators, Lisa Murkowski and Susan Collins, have both backed a delay in the vote until after November's presidential election.
If they are joined by two more Republican senators, they could block or at least delay a confirmation vote, as the Republicans have a majority of only six in the Senate.
In the event that the vote is a tie, the US constitution allows Vice-President Mike Pence to cast a tie-breaking vote.
Biden said that if he won the presidential election, Trump's nominee should be withdrawn. He said he would then consult senators from both parties before putting forward his choice.
He added that it would be wrong to release his list of potential Supreme Court nominees now, as this could expose some judges to political attacks.
But he said his first choice for the supreme court "will make history as the first African American woman on the court".
What has Trump said about Ginsburg's successor?
Trump has vowed to swear in Ginsburg's successor "without delay".
"I think it should be a woman because I actually like women much more than men," he said at a campaign rally in Fayetteville, North Carolina, on Saturday.
Earlier, Trump praised two female judges who serve on federal courts of appeals as possible choices. Both judges - Amy Coney Barrett and Barbara Lagoa - are conservatives who would tip the balance of the Supreme Court in favour of Republicans.
Democrats have vigorously opposed any nomination before November's election, arguing that Senate Republicans blocked Democratic President Barack Obama's choice for the US top court in 2016.
At the time, Senate Majority Leader Mitch McConnell justified the move on grounds that it was an election year. But on Friday Senator McConnell said he intended to act on any nomination Trump made and bring it to a vote in the Senate before election day.
The appointment of judges in the US is a political question which means the president gets to choose who is put forward. The Senate then votes to confirm - or reject - the choice.
Ginsburg, only the second-ever woman to sit on the Supreme Court, died of metastatic pancreatic cancer at her home in Washington DC, surrounded by her family.
Ginsburg, who served for 27 years, was one of only four liberals on the nine-seat bench. Her death means that, should the Republicans get the vote through, the balance of power would shift decisively towards the conservatives.
What does the Supreme Court do?
The highest court in the US is often the final word on highly contentious laws, disputes between states and the federal government, and final appeals to stay executions.
In recent years, the court has expanded gay marriage to all 50 states, allowed for President Trump's travel ban to be put in place, and delayed a US plan to cut carbon emissions while appeals went forward.
It is also deals with issues like reproductive rights - one of the main reasons some pro-life conservatives want to tip the balance away from liberals.
Who are seen as top contenders?
Barbara Lagoa: A Cuban American of the Atlanta-based 11th Circuit Court of Appeals, she was the first Hispanic judge on the Florida Supreme Court. She is a former federal prosecutor
Amy Coney Barrett: Member of the Chicago-based 7th Circuit Court of Appeals, she is a favourite of religious conservatives and known for her anti-abortion views. She was a legal scholar at Notre Dame Law School in Indiana
Kate Comerford Todd: Deputy White House Counsel, has a lot of support inside the White House. Served as former senior VP and Chief Counsel, US Chamber Litigation Center