At informal Washington dinners, on the floor of the House and on activist-filled conference calls, left-leaning officials are deliberating about how to forge an eventual alliance between Mr. Sanders, of Vermont, and Ms. Warren, of Massachusetts. Some are urging them to form a unity ticket, others want each to stay in the race through the primary season to amass a combined “progressive majority” of delegates, and nearly every liberal leader is hoping the two septuagenarian senators and their supporters avoid criticizing each other and dividing the movement.

“It can’t be, One candidate is the true god or goddess and the others are just shills,” said Representative Pramila Jayapal of Washington, who backed Mr. Sanders in 2016 but has not yet endorsed anyone in this race. “It really has to be, We’re trying to strengthen them both.”

On Saturday, Robert Reich, a former secretary of labor, sent an email to members of the progressive group Democracy for America warning that if supporters of the two candidates “wage war on each other” that it would “take both of them down.”

Former Senator Harry Reid of Nevada, one of the few prominent Democrats who commands the respect of both Mr. Sanders and Ms. Warren, said he was ready to step in if needed.