Senators as Presidential Candidates
One problem with this analysis is that we are dealing with a very small sample size, and each election is decided by multiple factors. Being a Senator was often not the deciding factor in the case of many losing candidates. Many of the recent Senators running for President have been running against a sitting President, which is generally an uphill battle. In recent years this included John Kerry, Bob Dole, Walter Mondale, and George McGovern.
Yes, it is true that three former Governors did beat sitting Presidents recently, but each was under unusual circumstances. Gerald Ford did not have the usual advantages of incumbency, and was running under the shadow of Watergate. Jimmy Carter was faced with two obstacles to reelection--the hostages in Iran and an unusually effective campaigner in Ronald Reagan. George Bush Sr. was elected only due to Ronald Reagan's popularity. As he subsequently became one of the worst Presidents in American history, foreshadowing his son's terms in office, it is not surprising that he was beaten by a Democrat who matched Reagan in charisma.
Historical factors reduce the long term significance of this trend. Until the 17th amendment was passed in 1913, Senators were appointed rather than being elected. It is possible that the lack of experience in running for a major office placed them at a disadvantage compared to Governors. In eras when the federal government was smaller and foreign policy concerns were not as great, a Governor also seemed the more obvious choice over someone from a legislative body. This is no longer the case. For example, while Howard Dean would be an excellent choice for running any state's Medicaid program, John Kerry showed far more understanding of national health care policy and the Medicare program. Similarly I listened to a recent health care symposium with Mark Warner and did not feel he approached the expertise of John Kerry (or any other Senator involved in health care policy) on national programs. The difference in expertise between Senators and Governors might be even more important politically over national security issues. Many question if Bill Clinton could have been elected after the 9/11 terrorist attacks, and I also note that his wife chose the Senate as her next step in hopes of returning to the White House.
Perhaps the most important factor which changes the disadvantage of a Senator is the nature of current campaigns. There is no doubt that there were disadvantages to being a Senator with hundreds of votes ready for misinterpretation. Often the meaning of a vote is completely different from what would appear from a brief description, as we saw with the Iraq War Resolution vote. It is easy for an opponent to take votes for or against a final budget to claim that a Senator supported or opposed everything in the final budget. Different circumstances and different details in different bills make it easy to fabricate charges of flip flopping.
What has changed is that Governors are no less subject to such political attacks. The Dukakis campaign showed what could be done to a Governor. Future Governors running will have every decision ever made scrutinized by opposition researches and spread on the internet and by friendly media. There was good reason why both George Bush and Howard Dean sealed many of their records. If there isn't enough unfavorable material in the record, that no longer matters. The most harmful attacks on John Kerry coming from the Swift Boat Liars had nothing to do with his Senate record. Similar lies can (and most likely will) be fabricated against any candidate.
Perhaps the biggest obstacle to a Senator is the opposition to a Washington insider although this would not explain why former Vice Presidents, many of whom had been former Senators) were successful in being elected. This is likely to be a bigger obstacle to Senators from the party in power. While some Republican Senators might have a tougher time in 2008, unless they have successfully cultivated a reputation as an outsider, a Democrat who is known for criticizing the Republicans in power might have an easier time. It is no coincidence that the last Democrat to be elected from the Senate was John Kennedy, beating a Republican after eight years of Republican rule. Perhaps another Senator JFK from Massachusetts will repeat this on a second attempt.