Even before Mr. Sanders lost four states in Tuesday’s primaries, dealing a huge blow to his presidential hopes, Ms. Warren was reluctant to support him, these people said. The spirited presidential campaign caused some rifts between the two liberals, including their clash in January over whether Mr. Sanders once told her that a woman couldn’t be elected president in 2020, an episode that deeply troubled her. Her camp also viewed Mr. Sanders’s electoral standing as fading in recent weeks, raising doubts about whether an endorsement would be a lost cause…

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.