Why do experienced Senators lose presidential elections?

[pbrower2a]

Governors have records as administrators. Some Governorships are good analogues to the Presidency in that a Governor is in charge of a huge political entity with much diversity. Of course the States are not divided as federal entities, so that is an imperfect analogy.

Governors might have to deal with emergencies -- like earthquakes, floods, and storms. Such can show extreme competence (Christie, 2012?) or incompetence (Blanco, 2005?) in a Governor. Governors can shake things up, but if they shake things up the wrong way (Scott, Snyder) they can get into big trouble and wreck whatever image they came in with.

...It could be that long-term Senators become increasingly un-Presidential as they get accustomed to the Senate. Kennedy and Obama were short-time US Senators.  One might have thought that someone with long and varied service in government (George Voinovich was Mayor of Cleveland, Governor of Ohio, and a US Senator) would be ideal because one would have the trifecta of high public service. But every politician at a certain level has some individuality, and it could be that some pols are wrong for the time. No President had a longer and more diverse record of public service than did James Buchanan.