Arlen Specter horrified and outraged many of his constituents, to say nothing of non-Pennsylvanians, when he changed his party registration back to Democrat after a thirty-year stint as a Republican. The reason, if you can believe it, was basically that he wanted to keep his job.
You'd think people went into politics because they wanted to improve the lives of their fellow citizens or right some egregious injustices or whatever, but it turns out that many politicians just enjoy being powerful and not having to pay for stamps.
And once politicians are elected, hoo boy! Turns out they often prefer to be in the majority party, so that they can exercise some control over the legislative agenda and enjoy fun committee assignments, instead of sitting sadly on their thumbs and moaning about how the majority is "running roughshod" over the beleaguered minority party.
This explains, sort of, why Jim Jeffords made the switch from Republican to Independent and began caucusing with Democrats in 2001, effectively handing them control of the Senate. It also explains why Joe Lieberman would become a third-party candidate in 2006 after he lost the Democratic primaries, but still caucus with the majority-party Democrats after he won re-election. (Probably doesn't hurt that he still votes like a Democrat most of the time.)
Dems who accused Lieberman of being terribly venal and self-serving in 2006 should be equally hard on Arlen Specter in 2009, but of course they will not, because everybody knows what's going on here. Senators who have held onto their seats for decades tend not to want to relinquish them, particularly to more ideologically pure primary opponents whose chances at winning the statewide election are worse (or not a whole lot better) than their own.