GOP Candidates Face Off In High-stakes Debate In Arizona MESA, Arizona - Mitt Romney and Rick Santorum, who has emerged as Romney's leading challenger in the Republican presidential race, clashed ahead of a high-stakes debate Wednesday, the 20th and possibly final debate of the roller-coaster campaign to challenge President Barack Obama. The debate was being held in the southwestern state of Arizona six days before a primary there and a crucial contest in Romney's native state of Michigan. The industrial state is now a must-win for Romney, who won it when he ran in 2008 and had been expected to win there again. Now, however, Romney faces a surging Santorum, whose candidacy has rebounded in the two weeks since he won three contests on the same day. A victory in Michigan -- no matter who claims it -- would provide momentum in the state-by-state race ahead of the 10 contests held on the same day a week later, the huge battle known as Super Tuesday. A new AP-GfK poll, meanwhile, found Republicans remain about equally divided on whether they'd rather see former Massachusetts governor Romney or former Pennsylvania senator Santorum capture the nomination. Former House of Representatives Speaker Newt Gingrich and Texas Rep. Ron Paul lagged well behind. Obama, who has been seen as vulnerable on economic issues, has seen his approval ratings rise as the economy shows signs of recovery from the Great Recession. Obama tops 50 percent support when matched against each of the Republican candidates and holds a significant lead over each of them, according to the Associated Press-GfK poll. Notably, the survey showed the president dominating among independents, a group central to Obama's 2008 victory. The debate was happening amid indications it could be the last. Romney, Santorum and Paul decided to pull out of another joint event that had been set for Atlanta. In the hours leading to the event, Romney called for a 20 percent across-the-board cut in personal income taxes as part of a program he said would revitalize the economy and help create jobs. Aides provided scant details. Romney's proposal sharpened his differences with Obama, who favors allowing tax cuts enacted under former President George W. Bush to expire on higher incomes. Santorum campaigned at a conservative tea party gathering in Tucson, where he said Romney's new tax proposal largely mirrored one he had had already made. Romney's decision to unveil elements of a new economic program during the day coincided with a fresh call for a corporate tax cut from the Obama administration. In a statement, Obama said he was presenting "a framework that lowers the corporate tax rate and broadens the tax base in order to increase competitiveness for companies across the nation" in the name of fairness. Corporations would have to give up dozens of breaks and corporations with overseas operations would also face an unspecified minimum tax on their foreign earnings. Santorum's rise in the race has left Gingrich and Paul on the outside looking for a way in. The former House speaker has yet to recover from a campaign nosedive that began after he won the South Carolina primary on Jan. 21, and he is pinning his hopes on his home state of Georgia to begin a comeback on March 6. Gingrich's decision not to campaign in Michigan so far has allowed Santorum to compete against Romney without also having to fend off a rival for the votes of conservatives.