Senator McCain—by emphasizing bipartisanship and the deficit, supporting "comprehensive immigration reform" and campaign finance reform, and opposing "enhanced interrogation" that he equated to torture—stands for the conciliatory, consensus-building politics of George H.W. Bush and James Baker. That doesn't mean he won't attack his opponents; his I-don't-change-my-positions-in-even-numbered-years line sliced like a Ginzu. But it's not his preferred MO, and it usually comes in the form of a counterattack.
Governor Romney—by emphasizing "conservative principles," MBA management, doing whatever it takes to defend America, and cutely but savagely attacking his opponent with buzz words like "amnesty," "sanctity," "Kennedy," and "Feingold"—stands for the divide-and-conquer politics of George W. Bush and Karl Rove. He jumped at the chance to slam McCain—and even his positive message about three legs necessary for the Republican stool (strong families, strong military, strong economy) is an implicit jab at his opponents.
(Rudy Giuliani, by the way, is an interesting amalgam all his own: more hawkish than Wolfowitz on national security, as conservative as Forbes on the economy, but Evan Bayh on social issues. And, as his attempt at an "I paid for this microphone"/"You're no Jack Kennedy" moment with Ron Paul demonstrated, he would be absent from the stage were it not for 9/11.)
The larger question is: Having lost the Congress, having sustained a president with sub-30% approval ratings, having endured war and scandal and disappointment, will Republican primary voters believe now is the time to compromise, or now is the time to fight?