"I don't think Ron Paul represents the mainstream," Romney added, his criticism illustrating an attempt to bloody the Texas congressman who, like Romney, is in strong contention to win Tuesday's caucuses.
The libertarian-leaning Paul, campaigning on the western side of the state, didn't immediately respond.
Romney commented on Paul's candidacy in interviews with Fox News Channel and NBC News after appearing with New Jersey Gov. Chris Christie before several hundred people outside a grocery store near Des Moines.
Mindful not to ignore another key state, Romney was dashing to New Hampshire later Friday for a couple of appearances and rolled out a fresh TV ad there. He returns to Iowa on Saturday afternoon and plans to remain in the state through caucus night. It's an indication of just how confident his team has become in a possible victory in a state that derailed his candidacy four years ago.
There's reason for that optimism.
A new NBC-Marist poll shows Romney and Paul virtually tied for the lead among likely caucus-goers, with Rick Santorum surging into third, Rick Perry inching up to fourth and Newt Gingrich sliding to fifth. Michele Bachmann was at the back of the pack. Perhaps more striking, tea party supporters are essentially divided evenly across the Republican field.
The race, to be sure, remains fluid and many Iowans still are undecided.
But Romney was expressing increasing optimism as his rivals scrambled to win over cultural conservatives and religious voters who haven't rallied behind one candidate, possibly paving the way for a victory by the former Massachusetts governor who is seen as less consistent on issues this bloc holds dear.
"I'm not planning on being disappointed," Romney said, speaking about Tuesday's caucuses. "No thing is a sure thing at this stage, but it feels terrific."
He also defended negative ads by his allies that have pounded Gingrich and argued that the former House speaker has done more damage to himself than any ads have.
"Speaker Gingrich's numbers have gone down more in New Hampshire than they have here," Romney said.
Romney refused anew to commit to releasing his income tax returns, saying: "If I become president, why, then I can decide that."
Illustrating an all-out effort to win the caucuses, Romney implored Iowans in a video to show up at precinct caucuses Tuesday and vote for him.
At the early morning event, he was clearly energized by the crowd of several hundred people as he delivered his usual campaign speech focused on President Barack Obama. "He just played his 90th round of golf," Romney said, knocking the president for vacationing in Hawaii as the crowd stood on cold, light rain.
Christie, a Republican backed by the conservative and tea party wings of the GOP, playfully told the crowd to vote for Romney or "I will be back, Jersey style."
With four days to go, public and private polling show Romney and Paul in strong contention to win the caucuses, with coalitions of support cobbled together from across the Republican political spectrum and their get-out-the-vote operations - beefed up from their failed 2008 bids - at the ready. They're the only two with the money and the organizations necessary to ensure big turnouts on Tuesday.
Three others - Santorum, Perry and Gingrich - will have to rely largely on momentum to carry supporters to the caucuses. Each was working to convince fickle conservatives that he alone would satisfy those who yearn for a nominee who would adhere strictly to GOP orthodoxy.
Bachmann was working to convince backers that her cash-strapped campaign was not in disarray after a top Iowa supporter abandoned her to back Paul.
Associated Press writers Thomas Beaumont, Mike Glover, Kasie Hunt, Brian Bakst and Shannon McCaffrey contributed to this report.