MESA, Ariz. — Struggling for an edge, Mitt Romney and Rick Santorum campaigned Wednesday into the 20th and possibly final debate of the roller-coaster race for the Republican presidential nomination.
Newt Gingrich and Ron Paul rounded out the debate roster six days before primaries in Arizona and Michigan.
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. The top tax rate would drop from 35 percent to 28 percent, and some popular breaks would be scaled back for upper-income taxpayers. However, aides provided scant details.
"We've got to have more jobs, less debt and smaller government, they go together," Romney said in an appearance in nearby Chandler. "By lowering those marginal rates, we help businesses that pay at the individual tax rate to have more money so they can hire more people."
Romney's proposal sharpened his differences with President Barack Obama, who favors allowing tax cuts enacted under former President George W. Bush to expire on higher incomes.
Santorum, who has emerged as Romney's leading challenger in the Republican race, campaigned at a tea party gathering in Tucson, where he said his rival's new tax proposal largely mirrored one he had had already made.
"Welcome to the party, governor, it's great to have you along," he said.
The debate came as yet another pivot point approached in a campaign that already has had more than its share of them. With a decision by Romney, Santorum and Paul to pull out of another joint event that had been set for Atlanta, there were indications this debate could be the last.
While some public polls show a close race in Arizona, Romney's campaign seems confident of winning the state's primary next week, so much so that it hasn't aired any television commercials to date.
But the former Massachusetts governor faces an unexpectedly strong challenge in his home state of Michigan, where Santorum is hoping to spring an upset. Santorum's candidacy has rebounded in the two weeks since he won caucuses in Minnesota, Colorado and a non-binding primary in Missouri on the same day.
The result is a multi-million-dollar barrage of television commercials in Michigan in which the candidates and their allies swap accusations in hopes of tipping the race.
A victory in Michigan — no matter who claims it — would also provide momentum for the 10 primaries and caucuses a week later on Super Tuesday. In all, 518 Republican National Convention delegates are at stake between Feb. 28 and March 6, three times the number awarded in the states that have voted since the beginning of the year. It takes 1,144 to win the nomination.
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.
Romney, like other Republicans, said that was a tax hike in reality.
"We have a very different course set out," he said. "The president will take us in one direction. He wants to raise taxes. That will kill jobs in this country."
But in fact, according to Romney's aides, the Republican's proposal for personal income tax cuts would include reductions in existing tax breaks to avoid raising federal deficits, the same claim that the administration made for its corporate tax recommendations.
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.
His campaign announced plans Wednesday to buy 30-minute blocks of television time in upcoming primary and caucus states for an infomercial on reducing energy prices.
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.
Paul has yet to win any primaries or caucuses.
He has weighed in against Santorum, though, airing an ad in Michigan that challenges the former senator's claim of taking a conservative line against federal spending. The ad says Santorum voted to raise the debt limit five times, and also supported legislation that created a prescription drug benefit under Medicare.
Associated Press writers Kasie Hunt and Charles Babington contributed to this story.