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Elizabeth Warren endorsed presumptive Democratic presidential nominee Hillary Clinton Thursday and called on Democrats to unite behind her.

"I'm ready to get in this fight and work my heart out for Hillary Clinton to become the next president of the United States and to make sure that Donald Trump never gets any place close to the White House," Warren told MSNBC's Rachel Maddow.

Warren's endorsement comes after both President Barack Obama and Vice President Joe Biden backed the former secretary of state on Thursday.

"I don't think there's ever been someone so qualified to hold this office," Obama said.

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Warren, a liberal stalwart known for championing issues related to economic inequality, has been one of the few Democratic senators to withhold a formal presidential endorsement.

"I thought that the primary was really important and it was an opportunity for Democrats to get out there and show this is what it means to be a Democrat," Warren said when asked why she withheld her endorsement for Clinton or primary rival Bernie Sanders.

"What Bernie Sanders did was powerfully important. He ran a campaign from the heart.... He brought millions of people into the Democratic Party and for me, this is what it's all about," the Massachusetts senator added.

Warren had already delivered a blistering critique of Donald Trump Thursday evening during a speech in Washington, continuing her recent role as antagonist to the presumptive Republican presidential nominee.

A Sanders aide told CNN's Jeff Zeleny Thursday night that Sanders and Warren spoke by phone earlier in the evening after attempts to reach him in advance of her endorsement.

A source said the decision was made for Warren to endorse Clinton Thursday, on the heels of Obama's endorsement, "To make it more forceful for the overall goal."

The Republican National Committee wasted no time blasting Warren after news of the endorsement surfaced.

"By endorsing Hillary Clinton, Elizabeth Warren has shown herself to be a sellout," RNC spokeswoman Lindsay Walters said in a statement. "Whether it's the Wall Street speech transcripts she refuses to release, her ties to the fossil fuel industry, or coziness with big banks, Hillary Clinton represents everything Elizabeth Warren supposedly stands against."

In the television interview Thursday night, Warren batted down speculation of Clinton tapping her as the vice presidential nominee. Warren responded with a flat "no" when asked if she has had conversations with the Clinton campaign.

"I know there's been a lot of speculation about this," Warren said, adding "the truth is I love the work I do. I can't tell you who grateful I am to the people of Massachusetts who sent me here to wade into these fights."

Warren was asked about comments by former Pennsylvania Gov. Ed Rendell -- a Clinton supporter -- that she is "not ready to be commander-in-chief."

Queried on whether she thinks she is ready, Warren responded, "Yes, I do."

News Courtesy: www.cnn.com