A constant refrain from conservatives in the past few months has been that Mitt Romney’s presidential candidacy is merely a re-run of past failed runs by GOP moderates like Bob Dole and John McCain. According to their reasoning, a candidate like Romney can’t fire up the Republican base and get them to turn out (the conceit behind Karl Rove’s successful campaigns for George W. Bush) in large enough numbers to win in November. Even more than that, conservatives assert that more Americans will be attracted to a conviction conservative (i.e. Ronald Reagan) than to a wishy-washy Republican who tried to be all things to all people. Romney’s centrist appeal to independents is, they claim, a snare that has trapped the GOP into putting up certain losers.

While Romney and his supporters have treated this line of attack as more the result of self-interest by his rivals than genuine analysis, these are serious arguments, and the former Massachusetts governor’s chance to win the nomination will turn on his ability to answer such challenges. Even more importantly, with the rise of Rick Santorum, it must be conceded that Republicans now have someone who is, as Charles Krauthammer noted this week on Fox News, plausibly presidential and who might actually test the conservative thesis.

Conservatives are right to point out that without the enthusiastic support of his party’s base, Romney will be fatally handicapped in a general election. Though it is unfair to paint him, as some have done, as a closet liberal, Romney’s image as a technocratic problem-solver who eschews ideology is not unfair and has made him a hard sell for Tea Partiers and social conservatives. The belief in his electability rests on the assumption that the desire to unseat Barack Obama is so strong among conservatives that it will overwhelm their reluctance to back Romney. That makes Romney’s ability to win the support of independents and even some disillusioned centrist Democrats who would be unlikely to pull the lever for a hard-core conservative a formula for Republican victory in 2012.

The question Republicans must answer in the coming weeks and months is whether there is a different and equally appealing alternative path to a GOP win. The problem for those who were appalled at the idea of nominating a moderate like Romney was that flawed contenders like Newt Gingrich, Herman Cain and Rick Perry were fronting the conservative argument. Though Santorum also has liabilities, his serious approach to issues of governance and foreign policy as well as an exemplary personal life allows him to put himself forward as a candidate more in the conviction conservative mode of a Ronald Reagan or George W. Bush than Romney. Moreover, as David Brooks pointed out in a perceptive column in today’s New York Times, Santorum’s impassioned pitch to working-class voters fits exactly with the blueprint for GOP success in the past than Romney’s business executive image.

The potential flaw in this thesis however is that unlike the sunny Reagan or the amiable younger Bush, it is far from certain the preachy Santorum can connect with centrists. While both of those Republican presidents had impeccable social conservative credentials, they didn’t come across as fiery culture warriors the way Santorum does. Now that he’s out of Iowa, the former senator needs to work on scaling back the fire and brimstone and emphasize the compassionate element of his conservatism, as he did when he reacted movingly to criticisms of his family’s method of coping with the death of a child.

Given Santorum’s potential problems, it must be said that Romney still has the stronger argument for electability. But if Santorum can work on humanizing his image while somehow overcoming the enormous disadvantages he has in terms of money and organization to actually compete for the nomination, then it must be conceded he might also have a shot at beating Obama.