That evening, CNN released audio of a tense exchange between Sanders and Warren after the conclusion of the last presidential debate, on Tuesday in Iowa.

"I think you called me a liar on national TV," Warren told Sanders on stage, referring to a back-and-forth in which Warren accused Sanders of privately telling her a woman couldn’t win the presidency, which he denied.

"What?" Sanders said.

When Warren told him again, "I think you called me a liar on national TV," he responded, "You know, let's not do it right now. If you want to have that discussion, we'll have that discussion."

He added, “You called me a liar.”

The exchange inflamed supporters of both candidates, who already were on edge after POLITICO reported that Sanders’ campaign had been using unflattering talking points to describe Warren in interactions with Iowa caucus-goers.

Fearful of splintering the progressive vote, leaders of progressive groups have been talking about unity within the movement since last year. But recent tensions between Warren and Sanders elevated their sense of urgency.

The pledge Thursday includes groups that are supporting Sanders or Warren, or both, or who have not yet endorsed any candidate. Among its ranks are DFA, Our Revolution, Justice Democrats, Sunrise Movement and the Working Families Party.

The groups said in a prepared statement that they will ask members to sign onto a pledge to vote strategically for Sanders and Warren in their caucuses or primaries. They said that in Iowa the groups will be “working to ensure their members do not leave their Feb. 3 caucus site without casting their final vote for one of the top two progressives in the race.”

“When progressives fight each other, the establishment wins,” Charles Chamberlain, chairman of Democracy for America, said in a prepared statement. “We saw it in 2004 when progressives took each other out and John Kerry slipped through to win Iowa and then went on to lose in November to a very unpopular Republican incumbent. We’re determined to not let that happen again.”

Similarly, Alan Minsky and Donna Smith of the Progressive Democrats of America said that although their organization has endorsed Sanders, “we love Elizabeth Warren too.”

“At a time when progressive policies resonate with Americans across the political spectrum, we need a progressive Democratic presidential nominee for the sake of the country and the world; and in order to defeat Donald Trump,” they said.