"One of the people running for president thinks it's okay for Iran to have a nuclear weapon," Romney, the former Massachusetts governor, said in this eastern Iowa city in response to a question from the audience. "I don't."
It was the first time Romney has challenged Paul directly since the Texas congressman jumped in polls. Neither Romney nor Perry, the Texas governor, named Paul, but the target was clear.
"You don't have to vote for a candidate who will allow Iran to have a nuclear weapon to wipe Israel off the face of the earth. Because America will be next," Perry said in Urbandale, reiterating a line of argument from a day earlier.
"I'm here to say: You have a choice," Perry added.
As if in rebuttal, Paul's campaign launched a new television ad describing him as "principled, incorruptible, guided by faith and principle" and the man to restore the economy. "Politicians who supported bailouts and mandates, serial hypocrites and flip floppers can't clean up the mess," it says as photos of Newt Gingrich and Romney appear on screen.
The stepped-up criticism of Paul, the libertarian-leaning Republican, comes as surveys show he's in contention to win Tuesday's caucuses.
In recent days, conservative opponents including Perry and Minnesota Rep. Michele Bachmann have increased their criticism of Paul on social issues, foreign affairs and inflammatory comments in his decades-old newsletter. By tearing him down, they hope voters will give their campaigns another, closer look after a season marked by candidates who have risen quickly in public standing only to fall back down.
Gingrich, whose slide in surveys over the past week has come as Paul has risen, said Tuesday he couldn't vote for Paul if he were to become the GOP nominee and called his views "totally outside the mainstream of every decent American" during an interview with CNN.
Gingrich, the former House speaker, began Wednesday, the second day of his Iowa bus tour with a speech to about 200 people in the atrium of the Southbridge Mall in Mason City. He plugged his support for supply-side economics favored by President Ronald Reagan.
Gingrich said the primary is giving voters a "choice between a populist supply side approach ... and a much more timid Washington-centered approach that will not create jobs."
Bachmann, who was on the 86th stop of her tour of Iowa's 99 counties, criticized both of her rivals from Texas. She accused Perry of spending "27 years as a political insider." He was a Texas legislator and agriculture commissioner before becoming governor in 2001.
Bachmann said Paul would be "dangerous as president" because of his hands-off views on national security.
Paul, for his part, was meeting with supporters near Des Moines, his first visit to the state since before the campaigns went dark for the Christmas holiday. He planned a series of events over the next two days as he looked to take advantage of a burst of momentum.
A conservative, Paul commands strong allegiance from his supporters but appears to have little potential to expand his appeal and emerge as a serious challenger for the nomination. Yet he could complicate other candidates' pathway to the nomination.
His opponents were spreading out across the state to woo potential caucus-goers, many of whom are still undecided amid a flood of television and radio ads.
In Independence, former Pennsylvania Sen. Rick Santorum mingled with 25 people at a diner and touted his plan to give a tax break to businesses that bring their operations back to the United States.
He told diners: "Things are going great, we've got momentum." He began airing a new radio ad Wednesday that promotes his hardline opposition to abortion and describes him as a "father of seven, a home-schooler and a devoted husband for 21 years."
Romney kicked off a three-day bus tour in the eastern edge of the state, in Muscatine, and shook hands with an overflow crowd at Elly's Tea and Coffee House. The line to get in stretched into the street.
Beginning the day, Romney told Fox News Channel that he was only joking Monday when he criticized Gingrich's failure to earn a spot on the Virginia ballot as something out of the sitcom "I Love Lucy."
"I hope the speaker understands that was humor, and I'm happy to tell my humorous anecdote to him face to face," Romney said.
Gingrich on Tuesday challenged Romney to make the "I Love Lucy" comparison to Gingrich's face.
Perry, looking to recapture the enthusiasm that greeted his entry into the race in August, railed against Washington and Wall Street insiders as he met with conservatives for breakfast near Des Moines.
"Why should you settle for less than an authentic conservative who will fight for your views and your values without apologies?" he asked, delivering the core rationale for his candidacy.
The packed crowd of conservatives in Urbandale applauded as he pledged to champion a constitutional amendment to balance the federal budget, secure the border within a year and crack down on illegal immigration. He also said he would bring his faith with him into the Oval Office, a nod to the Christian conservatives who have strong sway in the nominating process.
Elliott reported from Urbandale. Associated Press writers Mike Glover in Independence, Brian Bakst in Creston, Shannon McCaffrey in Mason City and Charles Babington in Des Moines contributed to this report.