AMES, Iowa — It probably made sense on paper: invite presidential candidates to visit the Iowa Republican Party’s tent in the parking lot before the big Iowa vs. Iowa State football game on Saturday.

What planners did not anticipate was the portable mob scene that characterizes the candidacy of Donald J. Trump.

Arriving more than an hour late, Mr. Trump offered a speech of less than a minute on the state party’s stage. But that was beside the point, as star-struck supporters greeted him like a stadium rocker during a sprawling tailgate party before kickoff.

“Donald, you rock!” a young man shouted as Mr. Trump, encircled by a security team, walked through a parking lot.