History predicts that Sens. Hillary Clinton, Barack Obama, John McCain and Sam Brownback might as well give up now rather than waste their time or their supporters' money. Why? Senators historically stand little chance of gaining the White House -- especially if their opponent is an executive (president, vice president or governor).
In the 20th century there were only two exceptions to this rule: in 1920, and in 1960 when Sen. John Kennedy beat Vice President Richard Nixon. In 1964, 1972, 1996 and 2004 incumbent presidents defeated senators -- Gerald Ford, Jimmy Carter and George H.W. Bush
President Bush would tell you that incumbency is no guarantee of re-election, and their successful opponents were governors. In the other post-JFK elections, both candidates were executives. Party affiliation was irrelevant.
Simply put, the rule is: Executives trump senators.
The only executive who is a serious candidate for 2008 is the former mayor of New York, Rudy Giuliani.
Considering the size of New York's population and the complexity of its administration, the mayor of New York is equivalent to governors of many states. If Republicans are astute, and Democrats run true-to-form, guess who I think the next president will be?