Reagan - at the time he was nominated by the GOP in 1980, his experience in government was 8 years as Governor of California. He had run for President in 1976, losing a close primary race to an incumbent President, Gerald Ford.
GHW Bush - at the time he was nominated by the GOP in 1988, he had been a two-term Congressman, UN Ambassador, Director of the CIA, Ambassador to China, Chairman of the National Republican Party, and Vice-President for eight years.
Dole - at the time he was nominated in 1996, he had been a U.S. Senator from Kansas for 28 years, and before that a four-term Congressman.
GW Bush - at the time he was nominated in 2000, he had been Governor of Texas for six years.
McCain - at the time he was nominated in 2008, he had been Senator from Arizona for 22 years, and before that had been a two-term Congressman.
Looking at this list, a quick and dirty analysis leads me to conclude that every successful candidate over the past 32 years had some significant executive branch experience, either as a state governor or as Vice President. The unsuccessful candidates, meanwhile, had the longest governmental careers, but all in the legislature. Of the successful candidates, the two best, Reagan and GW Bush, both had relatively short political careers as Governors of large states.
So let's compare the current candidates. Santorum exclusively has experience in the legislative branch, although not as long as either Dole or McCain. But his experience in the Senate is also qualitatively somewhat different, as I don't see anything in his record that suggests that he "went native" and allied himself with liberals on issues, the way both Dole and McCain occasionally did.
Meanwhile, Romney's four years of executive experience as Governor of Massachusetts is both shorter than Reagan and GW Bush's, and in a smaller and less consequential state. Romney does have significantly more business experience than any GOP candidate, probably ever. (It's an inapt comparison, but you could make a case that Eisenhower's experience running the ETO during World War II was the only other time when the GOP has nominated a candidate whose primary life experience was not in politics.... although you could just as easily argue that running the ETO was a political position par excellence. It's also worth noting that this will be the first time when the GOP will nominate someone without at least some military experience... Reagan rose to the rank of Captain during World War II, but served exclusively stateside making training films, while GW Bush as we all know was a fighter pilot in the Texas Air National Guard. Neither Romney nor Santorum has any military background.)
I don't know what all this means. Maybe it means that Romney's resume really does make him better suited to run successfully. Maybe it means that both our candidates are pretty weak.
It is worth noting that either Santorum or Romney has resumes and experience that are substantially stronger than Barack Obama's was when he ran for President in2008.