As Publius says, you've got moderate groups in the House and Senate (the Blue Dogs, the Evan Bayh squad) that exist primarily to give political cover to Democratic Congresscritters and Senators from conservative areas. Is there any reason these people don't want to be criticized from the left? If the point is to show off that they're moderates, don't they want liberals cursing their names -- including the expert liberal name-cursers who write on blogs? If this is correct, "holding fire for a bit" will be counterproductive -- they'll just keep moving right until we start firing and they can look moderate. I suppose if you're Nancy Pelosi or somebody with real power, you shouldn't do anything brutal to them, though noisy public threats of brutality may help their cause.
So the smart play for people at our keyboards would just be: start firing at them no matter where they are, unless they've uncharacteristically staked out such a progressive position on some random issue that it would just cause confusion. They'll thank you for it in December of an even-numbered year.