MESA, Ariz.: After months on the sidelines, Rick Santorum finally gets his chance at center stage in a debate of Republican U.S. presidential candidates on Wednesday, as well as the increased scrutiny that comes with it.

Santorum, surging in opinion polls, is likely to face tough questions over his strong conservative views on social issues when he and rivals Mitt Romney, Newt Gingrich and Ron Paul gather for the 8 p.m. EST (0100 GMT Thursday) debate sponsored by CNN.

With the March 6 "Super Tuesday" nominating contests in 10 states approaching, this may be the last major opportunity for all the candidates to make their case before a national audience on why they should be the Republicans' choice to face Democratic President Barack Obama in the Nov. 6 election.

Former Massachusetts Governor Romney, suddenly finding himself chasing Santorum after running as the seemingly inevitable nominee, needs a breakthrough performance to halt the former Pennsylvania senator's surge and restore luster to his own campaign.

Romney proposed an overhaul of the U.S. tax system on Wednesday that would cut Americans' tax rates by 20 percent and limit deductions for the wealthy.

Santorum needs to build on his momentum going into the Arizona and Michigan primaries on Feb. 28 and pave the way for Super Tuesday.

"For Santorum, there are a lot of expectations," said Republican strategist Ron Bonjean. "Now that he has reached the superstar status, he has to perform very well. Any stumble by Santorum will be magnified because of his front-runner status. He's going to have to face these attacks head-on."

Santorum and Romney are in a close race in Michigan, according to opinion polls, with most recent surveys showing the two divided by 4 percentage points or fewer.

An NBC/Marist poll on Wednesday had Romney at 37 percent to Santorum at 35 percent among likely primary voters in Michigan, a statistical dead heat.

The NBC/Marist poll gave Romney a lead of 42 percent to 27 percent over Santorum in Arizona. A Time/CNN poll on Tuesday found Romney ahead by 36 percent to 32 percent there.

A Quinnipiac University poll also released on Wednesday showed Santorum up 9 percentage points nationally against his Republican rivals, at 35 percent compared to 26 percent for Romney. It showed Gingrich with 14 percent and Paul with 11 percent.

Former private equity executive Romney is likely to attack Santorum over backing spending bills during his years in the Senate until he was voted out in 2006.

Santorum was on the defensive in Arizona on Wednesday about his time in Congress.

"We went there and we exposed scandal after scandal - bipartisan scandal, bipartisan scandals where Republicans and Democrats were doing things to undermine the credibility of Washington, D.C.," he said.

WHAT HAPPENED TO GINGRICH?

Gingrich, himself a former front-runner who has a strong record in debates, has dropped in polls since Romney surprisingly got the better of him in two televised encounters in Florida last month.

The race to find a Republican presidential nominee has been the most chaotic in decades as many conservatives have sought to find someone other than Romney, who made his name in politics in heavily Democratic Massachusetts, as their candidate.

Gingrich, Rick Perry, Herman Cain and Michele Bachmann all had their time in the limelight as alternatives to Romney, only to tumble away. Twenty previous debates have been crucial in determining who stays on top and who drops.

Now it is Santorum's turn.

In recent days his views against birth control and in favor of home schooling over public schools have drawn media attention. He has questioned Obama's "theology," and suggested that the president's healthcare law encouraged abortion by requiring insurers to cover various prenatal tests used to identify abnormalities.

The back-and-forth on social issues has detracted from Republican attempts to keep the U.S. economy as the central focus of the campaign and provided Democrats ammunition to pronounce the Republican field as out of step with mainstream Americans.

Romney's struggle has given voice to some in the Republican establishment who may want to draft a latecomer to the 2012 race out of concern that nominating Santorum would make Obama's re-election far easier.

The pressure is on Romney to raise questions about Santorum's electability. Santorum has Romney on the run in Michigan, where Romney was born and his father served as governor. A loss there for Romney could be devastating.

"The outcome of the debate will be an indicator of whether or not the Republican establishment will be closer to hitting the panic button," Bonjean said.