MESA, Ariz. (AP) -- Mitt Romney and Rick Santorum have traded verbal punches about health care, spending earmarks and federal bailouts in what might have been the final Republican presidential candidates' debate.

Santorum, who has surged in the polls, got booed when he talked about voting for the No Child Left Behind law even though he opposed it.

With pivotal primaries in Arizona and Michigan just six days away, with 10 more contests a week later, Romney and Santorum sparred more aggressively than in past debates, sometimes talking over each other's answers.

All four Republicans, including Newt Gingrich and Ron Paul, attacked President Barack Obama for his handling of Iran and its attempt to develop a nuclear program.

All four also opposed the federal bailout of the auto industry in 2008 and 2009.

The topic was a natural for Wednesday night's debate in Mesa, Ariz.

Newt Gingrich says he would devote as many resources as necessary to securing the U.S.'s southern border and that he would build two separate fences at the border to make it more difficult to cross illegally. He also says he'd send thousands more employees of the Homeland Security Department to border areas.

Mitt Romney says he would put more resources into securing the border and support programs that make it more difficult for illegal immigrants to work in the U.S. He says Arizona's approach to illegal immigration is a "model" for the country.

Rick Santorum says he would also beef up efforts to combat illegal immigration.