WASHINGTON — Some of his top congressional supporters won’t commit to backing him if he runs for president again — and two may join the 2020 race themselves. A handful of former aides might work for other candidates. And Bernie Sanders’s initial standing in Iowa polls is well below the 49.6 percent he captured in nearly defeating Hillary Clinton there in 2016.

Mr. Sanders may have been the runner-up in the last Democratic primary, but instead of expanding his nucleus of support, in the fashion of most repeat candidates, the Vermont senator is struggling to retain even what he garnered two years ago, when he was far less of a political star than he is today.

“It’s not a given that I’m going to support Bernie just because I did before,” said Lucy Flores, a former Nevada assemblywoman. “There are going to be plenty of people to look at and to listen to. I’m currently open at this point, and I think the majority of people are.”

As Mr. Sanders considers a second bid for the White House, he and his advisers are grappling with the political reality that he would face a far different electoral landscape than in 2016. Rather than being the only progressive opponent to an establishment-backed front-runner, the Vermont Independent would join what may be the most crowded, fractured and uncertain Democratic primary in the last quarter-century.