Joe Biden has accused Donald Trump of an "abuse of power" over his plans to nominate a new US Supreme Court justice before the election.
Speaking in Philadelphia, Mr Biden said Mr Trump was abusing his position by taking the action so close to an election, when some states had already started early voting, and should wait until November 3 to see who wins the White House.
In his first extended comments on the issue Mr Biden said: "I believe voters are going to make it clear they will not stand for this abuse of power. To jam this nomination through the Senate is just an exercise in raw political power.
"President Trump has already made it clear this is about power. Pure and simple. This fight won’t be over until the Senate votes, if it does vote. Winning that vote, if it happens, is everything."
He added: "There is so much at stake - the right to health care, clean air and water, and equal pay for equal work, the rights of voters, immigrants, women, and workers."
Mr Trump's nominee - expected to be Amy Coney Barrett, 48, a devout Catholic, or Barbara Lagoa, 52, a Cuban-American - would increase the conservative majority on the nine-member court to 6-3.
It could lead to massive changes including the overturning of Roe v Wade, the 1973 ruling that legalised abortion, and the dismantling of Obamacare.
Mr Biden appealed directly to moderate Republican senators, calling on them not to approve Mr Trump's nominee before the election.
He said: "I appeal to those few Senate Republicans - the handful who will really decide what happens. Don’t vote to confirm anyone nominated under the circumstances President Trump has created. Don’t go there. Uphold your Constitutional duty, your conscience. Cool the flames that have been engulfing our country."
Mr Biden added: "Supreme Court decisions touch every part of our lives and our future.
"The last thing we need is to add a constitutional crisis that plunges us deeper into the abyss, deeper into the darkness. If we go down this path, it would cause irreversible damage."
Mr Trump's nominee would need to be confirmed by a majority in the Senate, which Republicans hold 53-47. But he already faced a growing rebellion within his own party.
On Sunday night Lisa Murkowski, a Republican senator from Alaska, said she opposed moving forward before the election. Susan Collins, a Republican senator from Maine, had already indicated the same.
In a further twist former astronaut Mark Kelly, a Democrat, looked set to win a special Senate election in Arizona, which would give Democrats another vote.
Petite in size, huge in impact and powerful in the Court, we mourn the loss of RBG.
Make no mistake: we will honor her legacy of protecting the people – especially those living with pre-existing conditions. pic.twitter.com/lF8cJUVd6u
— Nancy Pelosi (@TeamPelosi) September 20, 2020
Meanwhile, Nancy Pelosi, the Democrat Speaker of the House of Representatives, refused to rule out impeaching the president in order to delay the confirmation process for his nominee. That would make Mr Trump the only president to be impeached twice.
Ms Pelosi said: "We have our options. Protecting our democracy requires us to use every arrow in our quiver."
In an interview on Fox News on Sunday night Mr Trump again questioned Mr Biden's mental acuity, saying his opponent "doesn't know if he's alive".
Mr Trump said: "You know it and I know it, Joe doesn't know if he's alive.
"If you asked him that question without teleprompters I don't think he could answer the question. And it's a sad thing that's going on, the whole thing is a sad thing."
Referring to his opponent having called his own ticket "Harris-Biden", Mr Trump said: "Who would say what he said? You would think it's impossible unless there's something wrong."