(CNN) Sen. Elizabeth Warren is treading cautiously into 2016 waters, planning to put the full backing of her progressive star power behind Hillary Clinton -- but eager to avoid the appearance of pushing out Sen. Bernie Sanders.

Warren, according to sources familiar with her thinking, won't take any steps before she has had a chance to speak to Sanders -- who hails from the same progressive wing of the party that is skeptical of Clinton. And she is wary of angering her base, sensitive over concerns that the establishment is maneuvering to strong-arm Sanders, who wants to campaign through the last primary next week in the District of Columbia and potentially until the nominating convention in July.

So Warren, like the rest of Washington, is playing a waiting game. She will wait to hear Sanders' message after meeting Thursday with President Barack Obama and Senate Minority Leader Harry Reid -- and whether he begins to wrap up his campaign. And assuming Sanders drops out soon, Warren may swoop in right after.

"She takes her potential role in unifying the party seriously," a source familiar with her thinking said.

Getting on the same page

What will make Warren's calculation more complicated is if Sanders insists on taking his fight to the Philadelphia convention, potentially robbing the party of a month to unite behind Clinton and putting like-minded Democrats like Warren in a bind. Warren is concerned about the reaction from her network of grass-roots donors, a Democratic source familiar with her thinking said, one reason why she is proceeding cautiously.

The discussions are extremely sensitive, given that few want to anger Sanders at this late juncture of the primary campaign. But Warren's backing is viewed as important for Clinton, who needs the help of the liberal Massachusetts Democrat to win over young and progressive voters who are fervent Sanders backers but skeptical of their party's presumptive nominee.

Warren, whose populist anti-Wall Street message has galvanized the left, has had a cool relationship with Clinton -- but that has changed as of late. As Clinton has emerged as the likely nominee, Warren has played a bigger role attacking Donald Trump -- something she plans to do in a speech in Washington on Thursday. Both the Warren and Clinton camps have spoken increasingly as of late, a sign that they are getting on the same page, sources say.

Potential running mate?

Moreover, Warren's growing role as an anti-Trump surrogate has fueled talk that she could end up as Clinton's running mate, an idea that sources say Senate Minority Leader Harry Reid would endorse.

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Warren was one of just two Senate Democrats to not endorse Clinton before this week -- and she was the lone female Senate Democrat not to do so. Her move had sparked some concern within the Senate, as Democrats were eager for her to help Clinton fend off a spirited challenge from Sanders.

Still, the fact that Warren has been neutral could allow her to play a role as a peacemaker to help win over Sanders' supporters -- but only if she's not seen as overreaching.

Some Clinton backers say Warren already has been taking steps to unite the party by her attacks on Trump.

"I'm confident that Elizabeth Warren is already doing that, and is already working to make sure that we all come together," said Sen. Claire McCaskill, D-Missouri. "I think if she formally endorses or when she formally endorses or who she formally endorses is not as important as her keeping her eye on the prize, and clearly she is. The eye on the prize is exposing a reckless and risky reality TV star for the selfish narcissism that he represents to this country."