AMES, Iowa — It was hard to miss him at the homecoming parade.

The band was booming. The floats were floating. And there he was — in the middle of it all — hands waving, hair flying.

“Hi, Bernie!” someone shouted from the sidewalk.

“How you doing?” he shouted back.

Senator Bernie Sanders of Vermont has not yet announced he is running. But for several blocks in this college town on Sunday afternoon, he was certainly marching.

From the front of a group of Iowa State College Democrats, Mr. Sanders, widely assumed to be preparing another presidential candidacy, inspired cheers and chants, his mere presence whipping up the kind of political excitement this early-voting state knows so well.

But if Democratic euphoria was the theme of the day, Mr. Sanders also sounded a cautionary note about the party’s prospects in the midterm elections on Nov. 6.