But for now, Mr. Trump’s bluntness has become his prime attraction. “People are just loving his honesty and his not-politics-as-usual approach,” said Bob Vander Plaats, a pro-marriage social conservative, who managed Mr. Huckabee’s winning 2008 campaign in Iowa.

“There’s just something about him that is intriguing people, and I think it’s that he’s saying things that other people just think,” said Robert Haus, an Iowa GOP campaign operative.

Even some old-line, mainstream Republicans are singing his praises. “Trump is not my top pick for the nomination, but only fools would underestimate him,” said Fred Malek, a GOP fund-raiser and a finance co-chairman of Sen. John McCain’s 2008 presidential campaign. “His statements don’t seem poll-driven, and I appreciate his directness.”