INDIANOLA, Iowa—Republican presidential candidate Ron Paul is gaining in Iowa polls, and Micah Luethje helps explain why. The 33-year-old is part of one of the most extensive voter-outreach efforts in the state, devoting 20 hours a week, he says, to the Texas congressman's campaign.

But Mr. Luethje also is well-positioned to see the potential limits to Mr. Paul's popularity. When he phones voters to seek support, they often say they have ruled out Mr. Paul. The problem: Mr. Paul's anti-interventionist views on foreign policy.

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