Please explain to me why would liberals support Paul in the primaries? He's virtually the only candidate who can pull a coalition of voters together like Reagan did in 1980. If he's nominated, the Dims will lose a good third of their voting base (young adults, single professionals, Reagan Democrats fed up with Bush, etc) right there.
Except for withdrawing from Iraq (and even then, his reasons for withdrawing is not similar to the liberals' reasons), Paul has nothing in common with liberals or liberalism. He wouldn't use troops to help the Left's favorite humanitarian causes. He supports limiting the federal government and is pro-life.
It's an old trick, and as simple a strategy as it is old. In the primaries, supporters of the candidates who are not and obviously will never gain the lead, will sometimes try to tear down the opposition party rather than waste their vote on an also-ran. In this example, Democrats "cross over" and vote in the Republican primary. The whole idea is to give a weak candidate a boost at the ballot box, resulting in a Republican nominee that they believe the leading Democrat would easily trounce. Under the right conditions, this practice (called "raiding", IIRC) can certainly influence who makes it to the general election.
So it's not about the Democrats actually supporting Paul, it's about them using him in open primary states in hopes of knocking out another more threatening Republican. We know about Barry Manilow's contribution, but we don't know yet if that was a fluke - or if there are other big-name liberals doing the same. It might be interesting to check out the usual suspects (Streisand, Clooney, etc.) on Opensecrets to see whether there's a trend forming.