The emergence of John McCain as the Republican frontrunner makes this American election unique.
It looks as if this will be the first election in US history where the nominees of both parties are members of the Senate.
On only two previous occasions has a Senator proceeded directly to the White House - John Kennedy in 1960 and Warren Harding in 1920.
And it has been surprisingly rare for even one Senator to be a candidate in the general election. There was Stephen Douglas in 1860, Barry Goldwater in 1964, George McGovern in 1972 and John Kerry in 2004.
Technically Bob Dole doesn't count because he resigned his seat before the election in 1996.
The traditional theory about this striking feature of US elections? That Senators talk their own legislative language, dry, technical, boring and not emotionally in touch with voters. And that they become part of the Washington machine and vulnerable to an anti-Washington campaign (which interestingly is exactly how Mitt Romney is now planning to go).
You can see both these weaknesses in John McCain. Fortunately for him, he'll be fighting another Senator. And both his potential opponents exhibit the same weaknesses.