Republican presidential contenders Mitt Romney and Rick Perry on Wednesday assailed Ron Paul for saying the U.S. has no business bombing Iran to keep it from acquiring a nuclear weapon, drawing a sharp contrast with their rising rival as he returned to Iowa days before the lead-off caucuses.

"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."

Open gallery view Rick Perry attending a campaign stop at the Westside Conservative Club breakfast, in Urbandale, Iowa. Dec. 28, 2011. Credit: AP

It was the first time that Romney has challenged Paul directly since the Texas congressman jumped in polls. Neither he 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 commercial 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.

Speaking earlier this week, a former Paul aide indicated that the presidential hopeful held anti-Israel position, which included his belief that Israel should cease to exist as a Jewish state.

However, on Tuesday, another former aide stepped up to defend Paul's position, saying that the Republican representative wasn't anti-Israel but objected to any form of U.S. foreign aid.