The last time America elected a sitting Senator was in 1960, with John F. Kennedy (D-MA). His Vice-President was a Former Senate Majority Leader, Lyndon Baines Johnson (D-TX).
Personally, I am a big fan of Governors and Vice Presidents running for POTUS. I know you folks on the Left do not understand, but I liked Gov. Sarah Palin (R-AK) -- a lot more then I liked Senator McCain (R-AZ).
(The reason why I did not mention CEO's is because, unless they worked in Government, it would be a different animal then Private Business. They would have the same problems as One Term Senators)
What makes a Governor or Vice President ready to take on a position of POTUS that a Senator (especially a first term Senator) cannot?
A Governor's job is to run the mechanics of Government within their State. Occasionally, they have to co-ordinate with the Federal side, but all in all, it's like preparing for the Majors. If you think Governor's are bad, why are Louisiana and Alaska the only ones not suffering from the Fiscal crisis right now?
How about a Vice President? While they are underneath POTUS, the intelligent ones are "Ministers without Portfolios," Thus, they choose an issue to work within the Government to affect. Also, they are officially known as "President of the Senate," Sometimes casting an end to a tie vote or Lobbying the Senate for POTUS with the authority of the Office behind him or her.
Foreign Powers recognize the Office they inhabit, thus learning Executive Branch Foreign Affairs on the fly, to be ready in case of the worst possible scenario or if they get elected afterwords.
And Senators? They vote. They make temporary coalitions to be thrown away if an issue changes. They suck up to the national media (which has, in turn, been sucking up to the American Prospect). Senators hold hearings and ask questions under a limited time frame donated by their Committee Chairman. When they are on the Well of the Senate, they can do individual things to frustrate others, but unless they are in Leadership, they do not Lead.
However, if you are planning to run for the Senate, it is a good start to be President, but at least be a Governor or run for Governor or be a Vice President before you look at the top job. Also, don't be a One Termer in the Senate either.
And when it came to one term Senators, it wasn't President Kennedy who got Civil Rights passed, it was his non-Ivy League, balder successor who knew how to get the Bills through.
President Obama is a product of the Ivy League and a One term Senator. He never built a coalition or ran an Executive Branch Agency or ran a State (like his Predecessor). He had three months to transition (compared to 35 days for President Bush in 2000) and he still is being rolled by the House Speaker?
Unless he stands up to his own Party and it's oncoming "Culture of Corruption" (Senator Dodd [D-CT] anyone?), he will be a One term President with no new ideas.