Sens. Elizabeth Warren (D-MA) and Bernie Sanders (I-VT) have been called the two most progressive candidates of the more than 20 Democrats who are vying for the Party’s 2020 presidential nomination.

That may be why the two are said to have agreed back in December of last year, according to “multiple senior Democrats,” not to criticize one another while on the campaign trail, New York Magazine reported.

But that pact may be coming apart. the magazine reported:

Half a year of campaigning later, with the pair running second and third in the Democratic race to take on Donald Trump, that arrangement held. Then, on Wednesday, Politico shared a story on Wednesday by tweeting, “Centrists are coming around to Elizabeth Warren as an alternative to Bernie Sanders,” and Sanders responded by retweeting it from his account, noting, “The cat is out of the bag. The corporate wing of the Democratic Party is publicly ‘anybody but Bernie.’ They know our progressive agenda of Medicare for All, breaking up the big banks, taking on drug companies and raising wages is the real threat to the billionaire class.” Political Twitter perked up — here was a real shot in the war so many had so long expected — until Sanders went on CNN a few hours later to tell Chris Cuomo, “That tweet was not about Elizabeth Warren, at all.” But this was the second rapid escalation and cleanup of the month. On June 7, U.S. News & World Report published a story quoting an anonymous Sanders adviser hacking at Warren’s prospects: “Warren fundamentally fails a basic threshold question: Can she beat Trump? Look at the general election polling. She does the worst of all candidates tested. That’s the DNA test debacle. It just fundamentally killed her. People want somebody who can beat Trump. She loses that argument.” After it appeared, Faiz Shakir texted Kristen Orthman, his former colleague in Harry Reid’s office. This doesn’t reflect Bernie’s thoughts, or mine, or the campaign’s official position, Shakir, now Sanders’s campaign manager, told Orthman, now Warren’s communications director and one of her closest advisers.

New York magazine reported that there are other rumblings about the growing competitiveness between Sanders and Warren, including the former Barack Obama communications director who claimed Sanders’ recent speech promoting his brand of democratic socialism was inspired by Warren’s rise in the polls and “a pretty clear indication he is feeling the heat from Elizabeth Warren’s recent momentum.”

In its preview of Sanders’ democratic socialist speech, the Washington Post said it “could sharpen the contrast with Warren, who is increasingly battling Sanders for liberal voters.”

The Week reported that “Bernie Sanders’ socialism speech might have been more about Elizabeth Warren than Trump.”

“Bernie 2020 is now four months old, and as the senator’s team continues to wrestle with how, exactly, to shape the latest version of his appeal in a political landscape radically different from 2016’s, the campaign’s posture toward Warren — who Sanders used to call “my favorite senator,” and whose own consideration of a campaign in 2016 nearly kept Sanders from running, out of deference — remains one of its least settled matters,” New York magazine reported

The magazine noted another reality — after running as the clear second-place candidate after former vice president Joe Biden, Warren is not only surging but virtually tied with Sanders in polls in California and Texas and ahead of him in South Carolina and Nevada polls.

Despite all of this, Sanders praised Warren in an interview on CNN, New York Magazine reported.

“I think that there are certain number of people who would like to see a woman elected, and I understand that,” Sanders said on Wednesday. “There are people who would like to see somebody who was younger, and I understand that also.”

“There are a lot of factors out there,” Sanders said.

But Faiz Shakir, Sanders’ campaign manager, sums up Sanders’ strategy this way:

“None of this campaign is based in reaction to another candidate,” Shakir said. “It is all based out of telling voters that Bernie Sanders has a unique philosophy, vision, and theory of change.”

“And he’s gotta talk about that, and make [it] clear to voters,” said Shakir, who has worked for House Speaker Nancy Pelosi (D-CA) and former Senator Harry Reid and the leftwing blog Think Progress. “And hopefully they’ll understand that, and we’ll have to live with whatever the voters decide.”

Follow Penny Starr on Twitter