Democratic U.S. Sen. Elizabeth Warren said Friday she will run for a second term in 2018, signaling again that she plans to be one of her party's fiercest critics of Donald Trump and the Republican-led Congress.

Warren made the announcement - which had been expected - in a message to supporters.

"This isn't the fight we were expecting to fight," Warren wrote. "But this is the fight that's in front of us. And the people of Massachusetts didn't send me to Washington to roll over and play dead while Donald Trump and his team of billionaires, bigots and Wall Street bankers crush the working people of our Commonwealth and this country."

Warren, a former Harvard Law School professor, won the Senate seat in 2012 by beating incumbent Republican Sen. Scott Brown in her first political contest.

Potential GOP opponents in 2018 include former Boston Red Sox pitching star Curt Schilling, a strong Trump backer.

Warren said she expected both the upcoming fights in the Senate over the next two years and her re-election campaign to be "uglier and nastier than anything we've ever imagined," adding she would take nothing for granted.

Massachusetts Republican Party chairwoman Kirsten Hughes suggested Warren was starting her campaign early because she had a lot of work to do.

"Her record has exposed her as a hyperpartisan bully more interested in scoring political points than delivering actual results, and Massachusetts has little to gain by sending her back to Washington," Hughes said.

Warren was among Democrats vetted by Hillary Clinton for the party's vice presidential nomination, which went to Virginia Sen. Tim Kaine. Warren has also figured in very early speculation about the 2020 presidential race.

In a recent interview with The Associated Press, Warren did not rule out a future White House run but added that it wasn't something she was thinking about at the moment.

She was recently named to the Senate Armed Services Committee, a high-profile assignment that could enhance her foreign policy credentials.

Warren appealed to supporters to make small donations to the campaign and volunteer to knock on doors or make phone calls on her behalf.

"We cannot and will not allow the Republicans and the powerful interests to sink our campaign the same way they sank so many campaigns in 2016," she wrote.

Warren reported more than $4.1 million in her campaign account as of Sept. 30, according to Federal Election Commission records, and she said previously her political action committee helped raise more than $5 million for Democratic candidates last year.

Warren assailed Trump during the 2016 campaign, calling him "fraudster-in-chief," among other things. Trump responded via Twitter, labeling her "goofy Elizabeth Warren" and referring to her as "Pocahontas," a reference to Warren's claim to have Native American ancestry.