Elizabeth Warren and Hillary Clinton met privately Friday morning, according to two knowledgeable Democrats, just hours after the Massachusetts senator formally endorsed the presumptive Democratic nominee.

The sit-down is fueling speculation about Warren’s prospects as a potential vice presidential pick.

The women have had several conversations over the past month, including one that lasted around half an hour, sources told The Washington Post. The conversations were broad and focused on large topics and issues, rather than the nitty-gritty of the campaign. Their staffs have been engaged in more tactical discussions.

Friday’s meeting took place at Clinton’s home in Washington and lasted about an hour with Warren departing around 11:45 am.

The two women do not have a particularly deep relationship, but that could change as Clinton rallies Democrats around her in the wake of winning the Democratic nomination in recent days. There were three big endorsements that could have meaningfully helped Clinton wrap up the nomination battle: Warren, President Obama and Vice President Biden. Clinton secured all three on Thursday.

Clinton, a Yale-educated lawyer, like Warren, a Harvard Law professor until she was elected in 2012, is a policy wonk at heart. So the two might talk in more detail about how Clinton could embrace pieces of the progressive agenda that allowed Bernie Sanders to win more than 20 states.

Warren is late to get on the Clinton bandwagon, but her support is crucial nonetheless to Democratic unity efforts. When all of the Democratic women in the Senate endorsed Clinton very early in the campaign,Warren was the lone holdout. She had earlier been the target of efforts from the left to draft her into the presidential race as an alternative to Clinton.

The Massachusetts senator justified staying on the sidelines until now by telling Rachel Maddow on MSNBC Thursday night that she thought it was “really important” to let voters choose in the primary and that the debate has been “constructive.”

“Hillary Clinton won,” Warren said. “And she won because she’s a fighter.”

“What Bernie Sanders did was just powerfully important,” she added. “He ran a campaign from the heart. … And he brought millions of people into the Democratic Party.

The meeting on Friday will only further fuel speculation about Clinton drafting Warren as her running mate on a historic all-female ticket.

Clinton campaign press secretary Brian Fallon declined to comment.

The vice presidential nominee historically plays the attack dog role on the national ticket, going hard after the other side so that the nominee can stay more positive. Warren has shown repeatedly in recent weeks that she is willing to throw punches at the presumptive Republican nominee Donald Trump.

Elizabeth Warren (D-Mass.) slammed Republican presidential candidate Donald Trump at the American Constitution Society for Law and Policy’s 2016 convention. (Reuters)

Just before formally endorsing Clinton, Warren delivered an anti-Trump stem-winder at the American Constitution Society convention in Washington. She told the liberal legal group that Trump is a “thin-skinned, racist bully” and “a guy who inherited a fortune and kept it rolling along by cheating people.”

A few weeks ago, she fired off a storm of tweets at Trump. They successfully got under Trump’s skin, which was the goal. He responded by attacking her, not just on Twitter but at his rallies. This further elevated her profile.

Since then, there have been more frequent conversation between the Warren and Clinton camps, below the level of the principals. Clinton’s team was very pleased with Warren’s first attack and offered encouragement. The Clinton campaign also gave Warren a heads up before her own speech attacking Trump in San Diego last week, outlining the key themes.

Warren is very clearly intrigued by the possibility of becoming vice president, but Friday’s meeting is not necessarily some kind of interview for the job.

Warren told Maddow that she is not being vetted. “I know there’s been a lot of speculation about this,” Warren said. “But the truth is, I love the work I do. I can’t tell you how grateful I am to the people of Massachusetts who sent me here to just wade into these fights.”

Friday’s session could be a way for Clinton to signal to the progressive wing of the party that she still cares about their priorities even after vanquishing Sanders.

Whether Clinton picks Warren, or a populist progressive in her mold,  ultimately depends on how much work she believes she must do to win over those who supported Sanders during the nominating contest.

Clinton may decide that it makes more sense to tack a little more to the center now that the primaries are over or that she should choose someone who could help deliver a battleground, such as Virginia Sen. Tim Kaine.

Trump’s racially-loaded attack on a federal judge of Mexican descent is just the latest example of an issue that might allow Clinton to make inroads with moderates, independents and even GOP-leaning women. Warren might not help bring those voters into the fold as much as someone else could.

Ed Rendell, the former chairman of the Democratic National Committee and governor of Pennsylvania, predicted earlier this week that Clinton would not choose Warren as her vice presidential nominee because the senator is not ready.

“I think Elizabeth Warren is a wonderful, bright, passionate person, but with no experience in foreign affairs and not in any way, shape or form ready to be commander-in-chief,” Rendell, a longtime Clinton ally, told a Philadelphia radio station.

The ensuing kerfuffle prompted Clinton to come to Warren’s defense. “I have the highest regard for Sen. Warren,” the former secretary of state told Politico. “I think she is an incredible public servant, eminently qualified for any role. I look forward to working with her on behalf of not only the campaign and her very effective critique of Trump, but also on the issues that she and I both care about.”

Maddow asked Warren about the Rendell quote Thursday night and whether she thinks she’s capable of being commander-in-chief. “Yes, I do,” Warren replied.

Sen. Harry Reid (D-Nev.) also thinks highly of the idea of putting Warren on the ticket. The Senate minority leader has expressed public concerns about losing the Senate seat if that were to happen, because the Republican governor of Massachusetts would get to pick Warren’s successor until a special election is held. But he is now more comfortable with the idea.

Short of becoming Clinton’s running mate, Warren still has a very big role to play in bringing Sanders’s supporters around and making them more enthusiastic about Clinton. That could be a big theme of the Friday’s conversation.

Republicans take Warren very seriously, and a constellation of groups attacked her when news of her endorsement broke. Both the Republican National Committee and American Crossroads called Warren a “sellout.”

Some significant differences exist between Warren and Clinton. In an old book, Warren attacked Clinton for changing her vote on a bankruptcy bill when she was a senator from New York, accusing her of siding with the financial services industry over consumers in order to get campaign contributions.

Warren has been Wall Street’s most outspoken antagonist in Congress. Sanders has benefited enormously from criticizing Clinton’s ties to the big banks and for getting hefty fees to speak at Goldman Sachs. The Vermonter has spent several months pushing Clinton to release the transcripts of her speeches to Goldman.

Warren ducked when The Boston Globe, her hometown paper, asked her Thursday night whether the woman she had just endorsed should release the transcripts. “That’s for her to decide,” she said. “There will be a whole lot of issues to talk about over the next several months.”

Hillary Clinton on the campaign trail

CLEVELAND, OH - On the third day of a bus tour through Pennsylvania and Ohio, Democratic Nominee for President of the United States former Secretary of State Hillary Clinton along with running mate Senator Tim Kaine, and Anne Holton, aboard the campaign bus in Cleveland, Ohio on Sunday July 31, 2016. (Melina Mara/The Washington Post)

Karen Tumulty and Anne Gearan contributed to this report.