Before he decided to run for president in 2016, Sen. Bernie Sanders (I-Vt.) famously waited to see whether his friend and progressive ally, Sen. Elizabeth Warren (D-Mass.), would do so first.

Now, following Warren's announcement Thursday that she's exiting the 2020 race, Sanders is waiting to see whether she'll endorse his candidacy — and unite the progressive wing of the Democratic Party behind his campaign.

Speaking Thursday at Burlington International Airport, Sanders confirmed that he had spoken to Warren for a second day in row since her disappointing finish in the Super Tuesday primaries. He would not say whether he had asked for her support, but he characterized their conversation as "positive" and praised her candidacy.

"You know, I've been in politics for a little while, and I have seen many, many campaigns — including campaigns that have spent a lot of money — kind of fade away. The message fades away the day after the candidate drops out or loses," Sanders said. "That will not be the case with Sen. Warren."

Warren, he continued, had run "a very strong, issue-oriented campaign, and the reason that her campaign ideas will remain viable for many, many years is she has changed political consciousness in America, which at the end of the day is the most important thing that any candidate can do."

Warren has not made clear whether she will endorse either of the major candidates remaining in the Democratic primary: Sanders or former vice president Joe Biden.

"Let's take a deep breath and spend a little time on that. We don't have to decide right this minute," Warren told reporters Thursday outside her home in Cambridge, Mass. "I want to take a little time to think a little more."

Warren and Sanders have been friendly for years, and for much of the 2020 primary they avoided publicly criticizing one another. That changed early this year after CNN reported that Sanders had said in a private meeting with Warren that a woman could not win the presidency in 2020. Warren confirmed the story, but Sanders denied it.

In the closing weeks of her candidacy, Warren publicly questioned Sanders' efficacy in the Senate, and Sanders campaigned in her home state on the eve of Super Tuesday.

In the end, Biden beat both of them in Massachusetts.