An unswerving loyalty to Paul, not to the Republican Party, is not uncommon among the congressman's passionate followers. His support is not easily transferrable. So while there's no doubt Paul brings new voters into the fold, it's unclear whether in the long term he is helping to build a broader and more diverse Republican coalition.

That presents a party trying to unseat a sitting president with a serious challenge, as Paul sits atop the polls in Iowa with a motivated grassroots army at the ready.

"It's problematic for people who are trying to expand the party, and it's probably true of Paul more than any other candidate because he has such outsider appeal," said John Stineman, an Iowa-based Republican consultant.

Washington-based GOP operative John Feehery, who has advised top Capitol Hill leaders, wrote on his political blog this week: "Ron Paul represents a challenge to the Republican status quo. And whoever the Republican candidate turns out to be (and my guess is that it won't be Ron Paul) will have to tread carefully on the Paul question."

Other Republicans anxious to protect Iowa's first-in-the-nation nominating contest are concerned that a victory by such an unlikely candidate would make the state look irrelevant. A string of questionable Iowa verdicts -- Mike Huckabee winning the 2008 caucus, Michele Bachmann winning the 2011 straw poll in Ames, topped by Paul winning the 2012 caucuses -- would provide fodder to critics who say Iowa's Republican activists don't represent the party's mainstream.

Scrutiny of Paul has ramped up in recent days, as he has climbed into the top tier of candidates seeking the nomination. A number of news outlets have written about bulletins published under his name in the 1980s and 1990s that included derogatory statements about African-Americans, Jews, and homosexuals. Paul has disavowed the racist remarks.

In one respect, Paul is well positioned to survive the controversy, since his supporters tend to be so distrustful of criticism from the political establishment. Still, coverage about the newsletters as well as a more intense focus on his unorthodox foreign-policy views could spook enough Iowa Republicans to keep first place out of his reach.

"I admit it doesn't look good,'' said Andrew Ivers, 39, who was also at the Paul rally. "If there's a skeleton in the closet, it needs to be addressed forthrightly."

Paul is trying to counter the bad publicity with television ads condemning the "Washington machine.'' Gingrich is a "serial hypocrite,'' while Romney is a "flip-flopper."

The latest CNN/Time/ORC poll shows Paul just 3 percentage points behind the front-runner, Romney, which is within the survey's margin of error. Paul's support increased from 17 percent one month ago to 22 percent now. He scored a nice publicity boost on Wednesday when Bachmann's former Iowa chairman, state Sen. Kent Sorenson, unexpectedly switched his allegiances at the rally.