Those 14 Senators are some of the most conservative in the Democratic party and some of them often side with the Republicans on key issues, and/or play the middle to get more of whatever it is they want.
Remember how hard it was for Obama to pass health care reform when he had 59 Democrats and Joe Lieberman?
Thats what the Senate + the filibuster does to our politics, it grinds it to a halt.
Edit:
First line should read: Some of those 14 Senators are amongst the most conservative in the Democratic party.
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?
For the 10 smallest states by population, 14 of the 20 senators caucus with the Democrats.
I think a radical electoral rewrite like this would probably help the Democrats in the Senate, but not by as much as you might think.