No one has ever doubted Rick Santorum's social-conservative bona fides. Sowing doubts about his fiscal conservatism is perhaps all Mitt Romney can do now to stop the former Pennsylvania senator from stealing the Republican nomination out from under him.
Mr. Romney used Wednesday's GOP debate to paint his suddenly dangerous rival in next week's Michigan and Arizona primaries as a pork-barrel politician who spent his Senate career at the trough, running up the national debt just to bring the bacon back home.
"Spending grew some 80 per cent when he was in the Senate," Mr. Romney said of his rival's 12 years in the upper chamber until 2006. "While I was fighting to save the Olympics, [he was] fighting to save the bridge to nowhere."
Mr. Romney's attempt to portray Mr. Santorum as a prodigious earmarker – slipping costly pet projects into unrelated legislation – only partly worked. That was because Mr. Santorum had a good comeback, reminding Mr. Romney that the V-22 Osprey aircraft loved by defence hawks (of which the GOP has no shortage) grew out of an earmark.
If Mr. Romney was counting on Wednesday's debate to destabilize Mr. Santorum, who now leads the former Massachusetts governor in national polls of Republican voters, he was only partly successful. But he won where it counted -- in looking more presidential when Mr. Santorum looked sophomoric.
Mr. Santorum provided a persuasive defence of his time in Congress, noting that he was a key actor in leading welfare reform to fruition in the mid-1990s and championed an overhaul of Social Security during the George W. Bush administration.
Those were gutsy stands for a politician from Pennsylvania: "We had a strong record in a tough state to be a conservative."
"If you look at my record on spending – I've taken on entitlements, never having voted for an appropriation bill increase," Mr. Santorum added. "Gov. Romney raised $700-million in taxes and fees in Massachusetts. I've never raised taxes."
Newt Gingrich, who has fallen to single digits in the Michigan polls and barely cracks double digits in Arizona, failed to break through in the debate. But he provided the most unambiguous critique of the Obama administration's bailouts of General Motors and Chrysler, which remain unpopular nationally and even among Republicans in Michigan.
Mr. Gingrich called the terms of the bailouts, which saw the United Auto Workers union win stakes in both car makers, "an unprecedented violation of 200 years of bankruptcy law to pay off the UAW."
All four of the Republican presidential candidates are critical of the bailouts, putting them on equal footing in the Michigan GOP primary. But the eventual nominee will face a challenge in Michigan as he tries to defend that position in the general election against President Barack Obama.
Wednesday's CNN debate was held in Arizona. But with Mr. Romney appearing to hold a comfortable lead over Mr. Santorum in that state, the debate was likely more critical to determining the outcome of the primary in Michigan, where the two are neck-and-neck.
An NBC News/Marist poll released Wednesday gave Mr. Romney a two-percentage-point edge with 37 per cent support, compared to 35 per cent for Mr. Santorum. Texas congressman Ron Paul garnered 13 per cent support and Mr. Gingrich had 8 per cent.
Michigan was supposed to be a cakewalk for Mr. Romney. He was born and grew up in the state, where his father was a popular three-term governor. And he handily beat John McCain in the state's GOP primary in 2008.
Now he is fighting just to stay alive. Wednesday's debate likely helped him, if barely.