Recent history shows that none of the three Republican frontrunners—not Mitt Romney, not Rick Santorum, and certainly not Newt Gingrich—can possibly win the GOP nomination, let alone defeat Barack Obama in November.
Because none of them are current office-holders, holding day-jobs that put them in touch with the realities of governance. None of them function in the real world of political give-and-take and daily compromise. They're all retired politicos trolling for activist votes in low-turnout primaries, or hoping to get well-paid speaking gigs after inevitable defeat by appealing to the most adamant and ideological elements in the conservative coalition.
For a generation, every Presidential nominee, of both political parties, has been a sitting Senator, Governor, Vice President or President. Last time, John McCain was the only major GOP contender who actually held public office at the time of his campaign (and still does, proudly), making daily decisions that not only impact politics, but influence policy—on taxes, spending, foreign affairs, and social issues. In 2008, McCain's principal opponents were former governors (Romney and Mike Huckabee), or a former Mayor (Rudy Giuliani) and a former Senator (Fred Thompson). It's not surprising that McCain—in far closer touch with the beating heart of the actual, functioning political system—won the nomination in 2008, as did prior GOP nominees over the last seven election cycles.
George W. Bush prevailed as a recently re-elected governor of Texas in 2000, as his father won as the incumbent Vice President in 1988. Bob Dole earned selection as his party’s standard bearer in 1996 as reigning Senate Majority Leader, though he made the mistake of resigning his post on June, after he had locked up the nomination.
Among Democrats, their nominees Michael Dukakis (1988) and Bill Clinton were popular current governors, while John Kerry (2004) and Barack Obama (2008) were still-serving Senators. Al Gore ran as the incumbent Vice President, after holding elective office for 24 uninterrupted years since the age of 28.