Super Tuesday 2012: Barack Obama accuses Republican rivals of 'playing games' over Iran Mr Obama's remarks were a direct response to attacks made earlier in the day by Mitt Romney, the front-runner for the nomination, and his closest rival, Rick Santorum, in addresses to the influential American Israel Public Affairs Committee (AIPAC). Mr Romney said that the Obama administration's "naive outreach" had served only to embolden the Palestinians, allowing them to exploit divisions between the US and Israel over the timing of any potential military action against Iran. I will station multiple carriers and warships at Iran's door. I will stand with the Syrian people who are being mercilessly slaughtered. I know that the fall of Assad would not only be an important victory for liberty, but also a strategic blow to Tehran," he said. Rick Santorum, the ultraconservative Catholic who has a long record of advocating a tougher line with Iran, gave an even more fiery speech, promising to issue an ultimatum to Iran if he was elected President in November. We need to say to the Iranian government: 'The time is now. You will stop your nuclear production now,'" he said to roars of applause that eclipsed the reception given to Mr Obama when he addressed the same forum on Sunday. If "this ultimatum is not met," if they "do not tear down those facilities, we will tear down them ourselves," he concluded. Mr Romney and Mr Santorum are the leading contenders the Republican nomination race that entered a potentially decisive phase on Tuesday when ten states voted on who should fight Mr Obama in November. As voting got under way, Republican leaders were hopeful that the marathon vote would bring some clarity to a contest that Barbara Bush, the former first lady of George Bush Snr, described as "the worst campaign I've ever seen in my life." The frequently vicious campaigning has seen Mr Romney's personal approval ratings with crucial independent voters sink to historic lows over the last month, even as Mr Obama's have risen on the back of a steady recovery in the US economy. Mrs Bush, now 86, has backed Mr Romney but said she was "sick" of the in-fighting which has damaged all the candidates. "I'm worried about this campaign, because it's too ugly, I really don't like it," she told Fox News. Mr Romney was expected to win Idaho, Massachusetts, Vermont and Virginia, while Mr Santorum looked safe in Oklahoma, but appeared to have lost a lead in Tennessee. Newt Gingrich was confident of victory in his home state of Georgia, while Ron Paul, the libertarian was campaigning for a first win of the primary season in Alaska. Analysts said victory Ohio for Mr Romney would put him in an almost unassailable position to win the nomination after a fortnight in which he has steadily facedown a challenge from Mr Santorum, an ultraconservative Catholic. While Mr Romney will remain far from the 1,144 delegates required to technically clinch the nomination, experts suggested that after Super Tuesday it would become too difficult for his rivals to win outright before the party convention in August.