Bernie Sanders can still get a crowd.

At rallies across the country before the midterms, from South Carolina to Michigan to Nevada, people came to see the Brooklyn-timbred senator from Vermont. They rattled the rafters in Wichita, Kan. They screamed his name in Iowa.

Yet as Mr. Sanders ponders another presidential run, what is not clear is whether he can still get the votes.

It is a question that has taken on new relevance as attention turns to the presidential election two years away: While the list of possible Democratic candidates seems to expand every day, political prognosticators have long assumed there would be a Bernie 2.0.

But if Mr. Sanders, 77, was a sensation in 2016, electrifying crowds and awakening fervor on the far left, he is no longer a singular figure among Democrats. Outflanked on the left by rising stars like Alexandria Ocasio-Cortez, Ayanna Pressley and Beto O’Rourke, his stronghold on the party’s progressive wing has weakened. Many of his key policy positions, including Medicare for All and tuition-free public college, have been embraced by others — a victory for him, he would argue, but one that makes his agenda seem less novel.