The United States Constitution says: "Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort." It would be a stretch, though perhaps not an impossible one, to say that Karl Rove is literally guilty of this.
But legal technicalities aside, outing CIA agents as part of a domestic political vendetta, at the possible cost of causing CIA contacts abroad to be murdered once foreign governments figure out who the American they had met with really was, certainly fits the spirit pretty nicely. Aid and comfort indeed.
Democrats reportedly are reluctant to turn up the rhetoric on Rove, or other Bush Administration shenanigans, too high. Thus, Howard Dean is told from time to time that he should back off.
If they studied the longstanding Republican playbook, they would rethink this a bit. What they need is someone to play the Agnew role, saying without mincing words that Rove is a traitor and that Bush knowingly tolerates traitors. Then a couple of more prominent Democrats can pooh-pooh this, saying, gee, he's not really quite a traitor, I think that's a bit too strong, etc.
Nixon, when he didn't have an Agnew, would be his own Agnew, describing some scurrilous accusation about a political foe, attributed to "some people say," so that he could then piously decline to endorse it himself.
As the saying goes, politics ain't pushpin. Since political warfare can be a tit for tat game, where you tailor your level of aggressiveness to what the other side is doing, the Democrats are right to think one generally shouldn't play ball this way. But against the Bush Administration it really is the only way to play.