CHARITON, Iowa — Around 10 p.m. Saturday, with locals here stuffing the entryway of a general store known for something called taco pizza, the presidential campaign of Senator Ted Cruz seemed to be winning a bet with itself.

Could the candidate fill the space on a frosty night, in a town of 4,000 or so, more than two months before voting begins, at an hour late enough that taco pizza was no longer being served?

“I’m pretty sure this is breaking a Guinness World Record,” Mr. Cruz said, settling in beside the doughnut display as scores of Iowans surrounded him. A burly man with a thick beard, a shaved head and a large soda offered to be Mr. Cruz’s “personal bouncer.”

“It’s a very bad idea in politics,” Mr. Cruz began, diplomatically, “to ask voters to leave.”

In fact, Mr. Cruz’s Iowa bandwagon, long churning in relative obscurity, seems to be approaching capacity. Event audiences have swelled. People are coming forward with endorsements. A recent Quinnipiac University poll of likely Republican caucusgoers gave Mr. Cruz 23 percent support, a close second to Donald J. Trump, in the clearest signal yet that Mr. Cruz has begun to tap into the outsider fervor that has for months powered rivals without public sector experience.