All sides are mobilising in response to US Supreme Court Justice Anthony Kennedy's retirement, promising a momentous confirmation battle for President Donald Trump's next nominee.
Senator Elizabeth Warren says the country is in a perilous state with Republicans controlling the executive and legislative branches, and Mr Trump about to make his second Supreme Court nomination.
The Massachusetts Democrat spoke at a rally Thursday in front of the Supreme Court, saying, "People around this country are worried and they're right to worry."
Ms Warren is considered a candidate to run for president in 2020.
She said a second Trump nominee on the court could jeopardise abortion rights, same-sex marriage, labour unions and environmental protections.
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Republicans already hold a slim majority in the Senate, so they should be able to approve any nominee as long as their coalition holds together.
But Ms Warren promised a spirited fight inside and outside the Senate.
"People are rising up. Donald Trump is not the king," she said. "We will fight today, we will fight tomorrow and we will fight until this country lives up to its values."
Thursday's rally attracted other likely 2020 candidates such as New Jersey Senator Cory Booker, New York Senator Kirsten Gillibrand and California Senator Kamala Harris.
Ms Gillibrand says that the retirement of Supreme Court Justice Anthony Kennedy has set up a situation where "women's lives are at risk".
The New York Democrat says that giving Mr Trump the chance to pick Kennedy's replacement threatens abortion rights and raises the question of "whether we are going to be arresting women for making decisions about their bodies".
Mr Booker is pledging a long-term battle to prevent Mr Trump from rushing a conservative judge onto the Supreme Court to replace Mr Kennedy.
The New Jersey Democrat told the crowd that, "We now must fight."
Mr Booker acknowledged that it will be difficult for Democrats to block any Trump nominee since Republicans control the Senate with a slim majority. But he said that American history is "a perennial testament to achieving the impossible".
Mr Trump says he will start the effort to replace Mr Kennedy "immediately" and will pick from a list of 25 names that he updated last year.
Senate Majority Leader Mitch McConnell says the Senate will vote on a successor this fall.
Democrats argue that a decision should be put off until after midterm elections.