Democratic senators refused to show up in protest at the GOP’s rush to install US President Donald Trump’s nominee to replace the late justice Ruth Bader Ginsburg.
The Senate has never before confirmed a Supreme Court nominee so close to a presidential election.
The Republicans, who hold the majority, voted unanimously in favour of Barrett, a conservative judge.
Instead of attending, Democrats displayed posters at their desks of Americans they say have benefited from the Affordable Care Act now being challenged in court.
Senators plan to convene a rare weekend session ahead of a final confirmation vote expected Monday.
“This is a groundbreaking, historic moment,” said Senator Lindsey Graham. “We did it.”
The 48-year-old judge’s ascent to the high court will lock a 6-3 conservative majority on the court for the foreseeable future.
Mr Trump’s Republican allies in the Senate are counting on her to improve their standing with voters.
The court could open a new era of rulings on the Affordable Care Act, abortion access and even the results of the presidential election.
Senate Democratic leader Chuck Schumer had announced the planned boycott in a speech late on Wednesday on the Senate floor.
“We should not be moving forward on this nomination,” he said, calling Ms Barrett’s views “so far out of the mainstream”.
Unable stop the confirmation, Democrats have been trying unsuccessfully to stall the process until after the November 3rd election so the winner of the presidency could name the new nominee.
“These are all such violations of American norms, values, decency and honour,” Mr Schumer said.
With Republicans holding a 53-47 majority in the Senate, Mr Trump’s pick for the court is almost certain to be confirmed.
All Democrats are expected to oppose Barrett’s confirmation.