Senators make the worst Presidential candidates IMO due to their voting history often get used against them in the General Election every time.
Obama and McCain both being Senators canceled each other out
usually Governors tend to cream Senators in the General Election.
"I actually did vote for it before I voting against it" LOL
There probably is something to this reasoning as a Senator one's record is quite a bit messier than a Governor's due to the nature of the two jobs. Empirical data would seem to back this up as well. If we discount 2008 since it was a Senator vs. Senator matchup, the last time a Senator won election was JFK in 1960. Since then (again discounting 2008) Senators have been nominated and lost in 1964, 1972, 1996, and 2004. During that same period Governors have been nominated and won in 1976, 1980, 1992, and 2000 while losing in 1988 and 2012. For the purposes of this analysis, I am counting Mitt Romney as a Governor because it was the political office he had held most recently while not counting, e.g., Richard Nixon as a Senator because he had been Vice President after between then and running for President.
But there's always the issue of having little data to go on and all kinds of confounding factors. Goldwater (1964) and McGovern (1972) held views that were well outside the mainstream of American politics. Also, McGovern and Dole (1996) were running against popular incumbents. On the other hand, Reagan (1980) and Clinton (1992) benefitted from running against unpopular incumbents. One could argue that running against another Senator allowed Obama to break the Senator "curse." On the other hand, given Obama's gifts as a politician and the conditions of the 2008 election, I would argue that he would've beaten any Republican that election. Incidentally, the last time a presidential election featured a Senator vs. a Governor was 1920 (the Senator won).
To make a long story short, I generally agree with your thesis, but I think the evidence is weaker than appears at first glance, and the effect isn't as large as sometimes supposed.