FILE PHOTO: Democratic 2020 U.S. presidential candidates (L-R) Senator Elizabeth Warren (D-MA) speaks with Senator Bernie Sanders (I-VT) as billionaire activist Tom Steyer listens after the seventh Democratic 2020 presidential debate at Drake University in Des Moines, Iowa, U.S., January 14, 2020. REUTERS/Shannon Stapleton/File Photo

NEW YORK (Reuters) - Liberal grassroots groups on Thursday launched a bid to calm tensions between supporters of the two progressive standard-bearers in the Democratic presidential race, U.S. Senators Bernie Sanders and Elizabeth Warren, who have engaged in a days-long feud after months of non-aggression.

Led by the political action committee Democracy for America, the 18 groups announced the “Progressives Unite 2020” campaign, which includes a promise to “focus our fight for the nomination against candidates supported by the corporate wing, instead of fighting each other.”

The spat between Warren and Sanders stems from a private conversation they had in December 2018 to discuss the 2020 race, when both senators were laying the groundwork for a presidential campaign to challenge incumbent Republican Donald Trump.

Warren has said Sanders told her he did not believe a woman could be elected president, a comment that Sanders has denied making.

The dispute intensified on Wednesday, when CNN aired previously unreleased audio of the two speaking after Tuesday night’s Democratic debate in Iowa. Warren, ignoring Sanders’ offer to shake hands, said, “I think you just called me a liar on national TV.”

Sanders responded that Warren, in fact, had called him a liar but said they should discuss the incident another time.

The new coalition includes groups that have backed Sanders, such as Our Revolution and the Sunrise Movement, as well as organizations supporting Warren, such as the Working Families Party and the Progressive Change Campaign Committee.

The groups said they would ask their members to sign a pledge to “vote strategically” in their states to ensure either Warren or Sanders emerges as the nominee. In other words, if their top choice falters, they would vote for the other liberal despite the current feud.

Warren and Sanders had informally agreed not to attack one another at the campaign’s outset, a truce that largely held until last weekend, when Politico reported that the Sanders campaign was training its canvassers to tell voters that Warren would not expand the party’s base.