The country’s largest nurses’ union will endorse Senator Bernie Sanders for president this week, a significant boost to his campaign from a major ally in the fight for his signature health care proposal.

The union, National Nurses United, fervently supported Mr. Sanders’s last bid for the White House in 2016, and its members have been significant players in Democratic politics since then, showing up in red T-shirts to support Mr. Sanders’s progressive allies in intraparty battles. They have also canvassed neighborhoods in swing congressional districts, urging voters to get behind “Medicare for all,” Mr. Sanders’s plan for a nationalized health insurance system.

“We know what we have done and what it takes to bring about fundamental change, and it’s massive organizing and a mass movement,” said Jean Ross, a co-president of the union. “Of all the candidates, Bernie is the one who understands that.”

The union plans to formally endorse Mr. Sanders on Friday at a news conference in Oakland, Calif.

After being so tied to Mr. Sanders in 2016, the nurses’ union was unlikely to endorse anyone else in the 2020 race. Mr. Sanders, of Vermont, was the only candidate to sit for an in-person interview with the union to seek its endorsement; Senator Elizabeth Warren of Massachusetts called in on a video chat and Mayor Pete Buttigieg of South Bend, Ind., sent a three-minute video. Union officials said they had invited former Vice President Joseph R. Biden Jr. and Senator Kamala Harris of California to interview but each declined to do so.