Washington, Nov 14: The leading Republican presidential candidates took hardline positions on U.S. foreign policy, with Mitt Romney promising in the latest debate that “Iran will not have a nuclear weapon” if he is elected to the White House.

Most of the candidates also, with the exception of Jon Huntsman, President Barack Obama's former ambassador to Beijing, vowed get-tough policies against China, the Asian powerhouse they said was stealing U.S. intellectual property, gaining trade advantage through currency manipulation and draining away American jobs.

While there were points of disagreement, the eight candidates at the Saturday night debate at Wofford College in Spartansburg, South Carolina, were unanimous in their criticism of Obama's handling of American policy abroad.

Romney's response was particularly striking when asked about Iran's nuclear program and a new U.N. report that backed U.S. and Western allegations that Tehran was making progress on building a nuclear weapon. Iran has insisted its nuclear program is in the peaceful pursuit of energy and research, not weaponry.

“If we re-elect Barack Obama, Iran will have a nuclear weapon. And if you elect Mitt Romney, Iran will not have a nuclear weapon,” the former Massachusetts governor said.

Romney has been at or near the top of the Republican field for months, but now is running neck-and-neck with Georgia businessman and political novice Herman Cain. Newt Gingrich, the former speaker of the House of Representatives, also has been gaining support. Both men have taken positions on foreign policy, the economy and social issues that generally are more extreme and find deep support among the conservative wing of the Republican party, especially with the highly motivated tea party faction.

Romney remains the favorite of the Republican establishment and has seen challengers to his pre-eminence rapidly rise and fade over the past few months. Congresswoman Michelle Bachmann of Minnesota was first in that line, followed by Texas Gov. Rick Perry. Cain was next but has been maintaining his standing despite allegations from four women that he sexually harassed them when he ran the National Restaurant Association in the 1990s.

Cain denies the allegations and claims donations to his campaign are rising because of what he claims is a smear campaign. Polls of Republican primary voters, however, show some of his support among women is fading. As that has happened, Gingrich has gained ground.

While the Republicans were talking about foreign policy, Obama was working as America's diplomat in chief.

Obama met with Russian President Dmitry Medvedev and Chinese President Hu Jintao on the sidelines of a Pacific Rim economic summit in Honolulu, prodding them for support in reining back Iran's nuclear ambitions but without winning public endorsement from either man.

The issue of Iran's nuclear program is fraught with danger because the Islamic regime in Tehran is harshly anti-Israel, a nation the United States has pledged to defend.

If the presidential trip gave the Republicans pause, they didn't show it in their 90-minute debate.

Romney, like all the others called for stiffer economic sanctions again Tehran, but did not rule out possible military action.

“It all else fails, if after all the work we've done there's nothing else we can do besides take military action, then of course you take military action,” he said.

Cain said he would not use the military but use more robust economic tools. Perry said the United States could shut down the Iranian central bank and bring the country to a standstill.

Gingrich joined Romney with the hardest line.

“There are a number of ways to be smart about Iran, and a few ways to be stupid. The administration skipped all the ways to be smart,” said Gingrich, adding later that “you have to take whatever steps are necessary to break its capacity to have a nuclear weapon.”

Even as Romney talks tough, conservative Republicans do not trust him. They see him as too moderate, view his Mormon religion as suspect and are angry about health care reforms that became law in Massachusetts under his leadership. That state's health care program served as a model for the national health care overhaul under Obama. Hardline Republicans have vowed to repeal the measure if they win the White House.

This debate, sponsored by CBS television and the National Journal, was the first to focus solely on foreign policy, which has been a secondary issue to the struggling U.S. economy in the campaign.

With few exceptions, the candidates signed on to Perry's declaration that he would start the U.S. foreign aid budget at zero for every country at the beginning of each year and only provide assistance to those countries—even Israel—that fell in line with American policy. Huntsman, Bachmann and former Pennsylvania Sen. Rick Santorum disagreed.

Perry also made light of his gaff in the last debate when he forgot the name of one of the three U.S. government agencies he planned to shut down.

The war in Afghanistan produced a wide range of responses, but unanimity in criticism of Obama's policy.

Huntsman said it was time to withdraw all U.S. troops from Afghanistan.

“I say it's time to come home,” he said. “This nation has achieved its key objective.” The libertarian-leaning Texas Rep. Ron Paul took a similar position on Afghanistan.

Romney and Perry said they would side with military commanders about when to withdraw troops.

Yet Romney backed a timetable of a complete withdrawal by the end of 2014, the same that Obama has cited.

Obama's would-be successors also differed on waterboarding, as well, the interrogation technique that former President George W. Bush authorized and Obama has banned. The practice simulates drowning and is viewed as torture by many.

While Cain and Bachmann both said they would reinstate the technique, Huntsman said its use diminishes U.S. standing in the world. Paul said it is illegal.