I am utterly opposed to the DLC. They are a divisive wedge in the party precisely at a time when we need unity the most. The DLC is unnecessary and damages the party.
However, the members of the DLC I support, in spite of the fact that I disagree with many of their policies.
I support DLC members this way. I write letters to them. I call their offices. I am an incessant nag. I engage them the way I do all representatives with a big "D" after their name, in a positive framework. I encourage them to support my liberal policies and thank them for support for any particular recent stance that I approved. If there is a vote I disagreed with I cite that as an example, again with as positive a frame as I can muster. It might be something like this:
"I was very disappointed in the Senator's vote on the Banking bill. Is the Senator aware of the blatant give-aways this gives to the large corporate banks? I certainly hope that the Senator takes this into consideration the next time something like this comes up for a vote."
Another tactic is to cite another Democrat's speech on the issue.
"I was particularly moved by Senator Reid's speech from the well of the Senate. I would have preferred that the Senator would have paid attention to the points Senator Reid made instead of listening to the opposition party."
A little bit of sugar helps get your point across without pissing off the staffer, or the Senator, which will get you precisely nowhere.
Remember, some of these people represent districts and states where a liberal could never be elected. It is a sufficient victory that they have a big "D" after their name and not an "R". That doesn't stop me from taking them on, but I don't slap the hand that feeds me.