As far as the effect on the DLC on the electoral fortunes of the Democratic Party, I think Nate Silver gets it just about right:
An alternative to suggesting that the D.L.C. was not all that successful, however, may simply be that it was not all that important. Surely, the D.L.C. had quite a bit of influence on Democratic strategy — the group was constantly in the news, and meetings between the group and Mr. Clinton were usually reported as high-stakes affairs. But political science research suggests that strategy, in general, matters only at the margin, with elections instead determined mainly by the economy, wars, scandals and other major events.One important caveat, though. The elections that Silver is talking about in that last sentence are general elections. Lots of things can influence primary elections. I don't want to speculate too much about the DLC in particular, but anything that mobilizes a large faction or group within a party that was previously unorganized can certainly affect nominations. At the presidential level, I really don't think that happened; no DLC, and I suspect that the nominations (Dukakis, Clinton, Gore, Kerry, Obama) turn out exactly the same. I suspect it did make a difference (perhaps at the recruitment level?) in down-ballot races, and in policy development, although others know more about that than I do (see, again, Kilgore's piece). Not as dramatic as dictating a nominee, or swinging a presidential election, but interesting and, perhaps, important nonetheless.