Those close to Ms. Warren say her foremost reason for not endorsing Mr. Sanders is simple: Since her exit from the race, his path to victory has looked unlikely. They doubt that Ms. Warren, even as the most prominent former candidate to have not backed another primary contender so far, could reverse Mr. Sanders’s fortunes at this point, and fear that she risks squandering valuable political capital if she tries to do so and fails.

It was also not clear what difference Ms. Warren might have made in addressing Mr. Sanders’s glaring vulnerability with black voters, with whom Ms. Warren had shown little sway herself.

Randi Weingarten, the president of the American Federation of Teachers, who endorsed Ms. Warren in her personal capacity, was among those who spoke with Ms. Warren after Super Tuesday.

“It made tremendous sense for her to stay on the sidelines so she could play the role of unifier,” said Ms. Weingarten, who declined to discuss her private conversation with Ms. Warren.

Brian Fallon, who worked for Hillary Clinton in 2016 and is now a progressive strategist, was doubtful that Ms. Warren’s backing would have significantly helped Mr. Sanders and said it might not have “sat well with the coalition she ended the race with,” which was dominated by college-educated white women.

“Why would she want to make her endorsement seem less powerful by giving it to somebody on a downward trajectory?” Mr. Fallon asked.

Four years ago, Ms. Warren stayed neutral in the Democratic primary between Mr. Sanders and Mrs. Clinton, and during the general election she used her influence among liberals to push Mrs. Clinton to make more left-leaning personnel choices in her transition team. “That was the template that she designed in 2016,” Mr. Fallon said. “Wait back, hold until the nomination is settled and then be very practical and hard-boiled about what your asks are.”