Sen. Elizabeth Warren announced on Monday that she was launching an exploratory committee for a presidential run in 2020.

In a video and a website that went live Monday morning, Warren described her vision of defending the middle class, which she said was "under attack."

The Massachusetts Democrat has long been expected to run, most recently stoking attention when she said she would seriously consider it after the 2018 midterm elections.

Sen. Elizabeth Warren announced on Monday that she had launched an exploratory committee for a presidential run in 2020.

In a video and a website that went live Monday morning, Warren described her vision of defending the middle class, which she said was "under attack."

Warren's campaign messaging appears to lean on her record of reforms and fighting big interests after she experienced economic hardship as a child and teenager in her native Oklahoma.

After a career in law and academia, Warren scrutinized large financial agents, using her work as a Harvard Law School professor to spur the creation of the Consumer Financial Protection Bureau and calling banking executives before the Senate Committee on Banking, Housing, and Urban Affairs, of which she's a member.

In the video released Monday morning, Warren pointed to her reform victories and her defeat of state Rep. Geoff Diehl, the Massachusetts co-chair of President Donald Trump's 2016 campaign, in her midterm election in November.

"I never thought I'd run for office, not in a million years," Warren said in the video. "But when Republican senators tried to sabotage the reforms and run me out of town, I went back to Massachusetts and ran against one of them. And I beat him."

The Massachusetts Democrat, who has long been expected to be a 2020 contender, recently stoked attention when she said she would seriously consider it after the 2018 midterm elections.

Read more: Meet the 2020 presidential contenders who are poised to start campaigning right away in 2019

Though Warren has already secured a large amount of financial support, a Boston Globe editorial earlier this month described low support for a presidential run among her constituents.

"Warren missed her moment in 2016, and there's reason to be skeptical of her prospective candidacy in 2020," The Globe's editorial board said, adding: "While Warren is an effective and impactful senator with an important voice nationally, she has become a divisive figure. A unifying voice is what the country needs now after the polarizing politics of Donald Trump."

Warren speaks at a campaign rally for Hillary Clinton, the 2016 Democratic presidential nominee, in Manchester, New Hampshire, on October 24, 2016. Thomson Reuters

Despite doubts about her chances in an already crowded group of Democratic contenders, Warren has stayed in the spotlight.

She and Trump earlier this year had a tense back-and-forth over her claims of Native American ancestry, and she released the results of a DNA test to a mixed reception.

Warren is the third Democrat to officially declare their interest in running in 2020; the other two are Rep. John Delaney of Maryland and Julián Castro, a former San Antonio mayor and adviser to President Barack Obama.

Other high-profile Democrats poised to announce their intentions for 2020 include Sen. Bernie Sanders of Vermont, Sen. Cory Booker of New Jersey, and Rep. Beto O'Rourke of Texas, all of whom have recently been making stops in key election states.