Here is a link to a New York Times article that I found interesting, regarding Connecticut Senator Joe Lieberman.
Lieberman was originally elected to the Senate as a Democrat, and was that party's unsuccessful vice-presidential nominee in 2000. He lost the Democratic primary for his Senate seat in 2006. He then won the general election, running as an independent.
He voted with the Democrats in organizing the Senate in the aftermath of the 2006 elections. He thereby helped to shore up the Democrats' narrow 51-49 majority in that body, along with one other independent New Englander, Vermont Senator Bernie Sanders. Lieberman thereby maintained his seniority within the party, allowing him to chair the Senate Committee on Homeland Security & Governmental Affairs.
This year, he has endorsed Republican Senator John McCain for president. The Times article goes into more detail about all that that entails.
Current polls indicate that the Democrats are likely to increase their Senate majority in November. If so, that party's majority status will no longer hinge on Lieberman's position. That could cause either Lieberman or his colleagues to change their relationship.
I wrote earlier about how the split between the Democratic and Republican parties is now more driven by ideology than has historically been the case. It still oversimplifies to call the Democrats the liberal party and the Republicans the conservative party, but that is now closer to being accurate. It seems to me that the Lieberman question largely boils down to: what does a political leader do when he agrees with his party's ideological position on many issues, but not on other important issues, such as the war in Iraq?