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“The issue for a lot of these caucus-goers is, ‘Do I go with my heart, a candidate who’s really pure and I love, or do I opt for somebody who I think has a chance of winning,’ ” Mr. Yepsen said.

Mr. Yepsen said Mr. Paul’s showing here sends a signal that his military-retrenchment views resonate with a significant chunk of the GOP’s voters. He also said the Iowa results will mean other candidates in the field will have to grapple with that fact.

“Regardless of what you think about Ron Paul, he’s got a message that’s resonating,” Mr. Yepsen said. “I think he’s going to do well enough, and there’s enough of a presence, that it’s a message that can’t be ignored.”

The entrance poll showed nearly two out of every five voters was a newcomer to the caucuses.

The polls also showed deep striations: Voters for whom abortion was the most important issue overwhelming supported Mr. Santorum, while Mr. Paul won a plurality of voters concerned about spending and Mr. Romney held a lead among the 42 percent of voters who told the poll they were concerned about the economy.

Mr. Romney led among suburban voters, Mr. Paul won urban voters and Mr. Santorum led in the rural vote.

Issues and geography aside, voters in Iowa said they are desperate for a champion who can beat Mr. Obama in November.

“I’ve never been involved before, and I feel like I’m like a lot of people who woke up and said, ‘We need to get Obama out of there,’ ” said Becky Ervin, 62. “We need to get the economy back on track, and Obamacare really scared me. I also don’t believe in spreading the wealth around — 50 percent of the country doesn’t pay taxes, and that’s not right. I just don’t like where the country is heading.”

Paige Winfield Cunningham and Susan Crabtree contributed to this article.