[Read more: Bernie Sanders drops out of 2020 democratic presidential race.]

The Center for Popular Democracy Action, a coalition of more than 40 progressive community groups totaling about 600,000 members, will on Tuesday endorse Senator Bernie Sanders of Vermont for the Democratic presidential nomination, signaling that Mr. Sanders’s political standing among the party’s left wing has rebounded — or even risen — in the months since he suffered a heart attack.

At the time of his hospitalization, in early October, Mr. Sanders was under intense political pressure. Senator Elizabeth Warren of Massachusetts had surged past him in polling and positioned herself as the progressive standard-bearer in the race, buoyed by an endorsement from the Working Families Party, a key progressive group that had endorsed Mr. Sanders in the 2016 race.

Now, their political fortunes have shifted. Mr. Sanders has risen in the polls, while Ms. Warren has plateaued, stymied by a barrage of political attacks that have forced her to make a rare shift in messaging. Leaders for C.P.D. Action, which includes liberal groups focusing on immigration, health care, housing and other issues, said Mr. Sanders had seen a jolt of popularity within the organization in the past two months, surpassing Ms. Warren. This is the first time the group, founded in 2012, has endorsed in the presidential race.

Jennifer Epps-Addison, the group’s co-executive director, called it a “resurgence.”

“What Bernie Sanders has built — it is not arguable,” Ms. Epps-Addison said. “It’s an exciting and different energy than we’ve seen in the Democratic base.”