"The Republican majority is lighting its credibility on fire ... The next time the American people give Democrats a majority in this chamber, you will have forfeited the right to tell us how to run that majority," Senate Minority Leader Chuck Schumer, D-N.Y., said during a floor speech Monday.
"My colleagues may regret this for a lot longer than they think," he added.
Nominees once needed 60 votes to be confirmed, but Sen. Mitch McConnell changed the standard in 2017 to allow for a simple majority. That move allowed for the confirmation of President Trump's previous two nominees, Justices Neil Gorsuch and Brett Kavanaugh.
Former Senate Majority Leader Harry Reid, D-Nev., first eliminated the 60-vote threshold in 2013 to overcome GOP stonewalling of President Obama's nominations to the lower courts and the executive branch. Known as invoking the "nuclear option" at the time, Reid kept the higher standard in place for the Supreme Court.
The comments by Schumer appeared to be similar to those made by McConnell back in 2013 after the Democratic-controlled chamber eliminated the 60-vote threshold.
“You’ll regret this, and you may regret it a lot sooner than you think,” McConnell said in 2013, according to the Hill.
House Speaker Nancy Pelosi also released a statement criticizing Trump and the GOP Senate for "committed an act of supreme desperation" so close to Election Day.
Pelosi argued the confirmation, which she called a manipulation, was made so Trump and Republicans could "achieve their years-long campaign to destroy Americans’ health care"
“The President’s Supreme Court manipulation threatens the very values and rights that define and distinguish our nation: a woman’s constitutional right to make her own medical decisions, the rights of LGBTQ Americans, the right of workers to organize and collectively bargain for fair wages, the future of our planet and environmental protections, voting rights and the right of every American to have a voice in our democracy," Pelosi wrote in a statement.
Democratic senators warned that Republicans have lost the right to complain if they win back the majority.
"Will Democrats go to new, extraordinary lengths to maximize their power given the extraordinary lengths Republicans have gone to maximize their power? This is not a conversation that is ripe enough yet, but what do Republicans expect?" said Sen. Chris Murphy D-Conn., as part of the chamber’s debate over Barrett.
"Do we just unilaterally stand down and not choose to use the same tools that Republicans did in the majority? ... I think there are now new rules in the Senate, and I think Republicans have set them," he continued, according to the paper.
Sen. Sheldon Whitehouse, D-R.I., released a statement calling the confirmation a "sad day for the Senate and for the Court."
"My Republican colleagues put the rule of ‘because we can’ over the traditions and precedents of the Senate, the principles we hold dear as an institution, and the integrity of the federal judiciary," he said.
The Supreme Court said in a press release Monday that Barrett will be able to start her new role after Chief Justice John Roberts administers her judicial oath on Tuesday. Justice Clarence Thomas administered the constitutional oath at Monday's ceremony.
Fox News' Ronn Blitzer and Edmund DeMarche contributed to this report