Senator Lieberman lost his right to run as a **Democrat** in the November election.
He can still run for election on the November General ballot, but to do so he will have to **switch** parties (which he did tonight).
But by switching Parties, he is no longer a Democratic Party Democrat. He's switched to some flavor of Independent Senator.
Thus, the Democratic Party has lost a sitting Senator tonight to a Party switch; they have only 43 left (plus 2 Independents: Lieberman and Jeffords).
I predict they he may be the first of many to become Independents.