Amy Coney Barrett, the new US Supreme Court justice, faced calls to recuse herself from a key legal case involving election ballots on her first day in the job.
The Supreme Court is considering a case brought by a Pennsylvania county over whether mail-in ballots received three days after the election can be counted.
Lawyers for the Luzerne County Board of Elections wrote that their motion "comes at an inopportune time for Justice Barrett, this being her first day". But they argued her involvement would be "catastrophic".
They wrote: "Just as President Trump has placed Justice Barrett on the Supreme Court with whatever hope or expectation he may have, he has also imposed on her the duty to recuse herself in this case. Her integrity and the integrity of this Court cannot tolerate any other choice."
Ms Barrett's elevation to the court means some of her first votes could be to do with disputed ballots if the US election in a week's time is contested.
She could also rule on a host of issues including healthcare, abortion, and guns
It brings the conservative majority on the nine-member court to 6-3, solidifying the ideological makeup of the court for years to come.
The Supreme Court confirmed that Ms Barrett will occupy the chambers formerly used by the late justice, and liberal icon, Ruth Bader Ginsburg.
Joe Biden attacked Ms Barrett's confirmation as "rushed and unprecedented," and warned it could lead to the end of Obamacare, also known as the Affordable Care Act.
He said: "Just a few days after Election Day the Supreme Court will hear the case on the Affordable Care Act. He (Mr Trump) wants to tear down the Affordable Care Act in its entirety and take away your healthcare. This goal - the goal of the Republican Party - was a litmus test in selecting this nominee."
Ms Barrett, 48, became the 115th justice in the court's 231-year history, and the third to be nominated in just four years by Mr Trump.
She was confirmed in the Senate by a vote of 52-48. All but one Republican senator - Susan Collins of Maine - voted for her. It made her the first Supreme Court justice in 150 years not to receive any support from the opposing party in the Senate.
At a swearing-in ceremony at the White House Mr Trump said: "This is a momentous day for America, for the United States Constitution and for the fair and impartial rule of law.
"She is a woman of unparalleled achievement, towering intellect, sterling credentials, and unyielding loyalty to the Constitution."
During her Senate confirmation hearings Ms Barrett faced questions from Democrats over how her strong Catholic faith might affect her decisions.
Speaking at the White House, she said: "It is the job of a judge to resist her policy preferences. It would be a dereliction of duty for her to give in to them.
She said it was necessary for a justice to "declare independence from private beliefs that might otherwise move her".
Ms Barrett said: "I will do my job without any fear or favour, and I will do so independently of both the political branches, and of my own preferences."
At the outdoor White House swearing-in ceremony, attended by 200 people, chairs were socially distanced and guests wore masks.
It looked markedly different to an event held there earlier this month to announce Ms Barrett's nomination. That became a "superspreader" event after numerous people contracted the coronavirus.
Elizabeth Warren, the Democrat senator, called the Senate vote "illegitimate" and "the last gasp of a desperate party."