My thesis is simply that more rural states tend towards more conservative politics and when Democrats get elected in rural states they tend to be the more conservative members of the Democratic party.

Ben Nelson is one of the best examples of this phenomenon. During the health care reform bill he was a holdout until the end, threatening his own parties signature piece of legislation. He caved when his state was given a special deal.

But small states aren't all rural, and even some of the ones that are elect fairly liberal Senators. Vermont is the classic case, but Democrats from the Dakotas have been almost legendarily liberal, too. Hell, Tom Daschle was Minority/Majority Leader.

And liberal Republicans get elected from small states, too. Remember Lincoln Chafee?