Here is a brief chart showing the highest previous office held by every President since 1904:
|2000||George W. Bush||Governor|
|1988||George HW Bush||VP|
|1960||John F Kennedy||Senator|
|1952||Dwight D Eisenhower||General|
|1948||Harry S Truman||VP/Pres|
|1928||Herbert Hoover||Sec'y of Commerce|
|1908||William H Taft||Sec'y of War|
Harding was the first sitting senator to ever become President.
With the exception of Nixon, all vice presidents who became president were incumbents (at least in this time period.)
It appears that Americans trust governors far, far more as leaders than they do senators. The numbers seem to indicate an almost visceral distaste for senators - when senators go up against governors,
This could partially explain the appeal of Sarah Palin - even though she is a first-term governor, so were Jimmy Carter and FDR (although they were in office longer.) Americans seem to trust governors more with the presidency than they do Washington insiders; I believe that Harding was the only senator to ever defeat a governor for president.