Senator Bernie Sanders offered a full-throated endorsement of Hillary Clinton on Tuesday, urging his supporters to get behind the presumptive Democratic presidential nominee. But many of his most loyal followers are not feeling it just yet.

Disappointment in Mr. Sanders cascaded across the internet as he embraced his former rival, describing her as a comrade in the fight to overhaul a rigged campaign finance system and lift the poor out of poverty. The sadness was most evident on the Facebook page where the Vermont senator explained his decision in a message titled “Forever forward” that drew responses infused with a skeptical refrain: Never Hillary.

“You broke my heart and betrayed the left Senator Sanders,” wrote Cesar Agusto Diaz, a Sanders supporter from New York.