I think this actually extends to opposition in general (See: Alito, Roberts, Bankruptcy Bill). By not standing up to an unpopular president's unpopular agenda, the Democrats are alienating the very people they are supposed to be appealing to. Despite what the media constantly tells us, Republicans are not the middle.
Let's just look at the numbers from my most recent national poll (July 21). Overall, only 36% of likely voters told us that they agree that the war in Iraq has been "worth the loss of American lives", while 57% disagree. But the partisan splits are more revealing: only 16% of the Democrats polled said the war has been worth while 82% disagree and only 26% of Independents agree the war has been worth it while 72% disagree. On the Republican side, 64% said the war has been worth it, while 23% disagree. The war has been the principal cause of the nation's polarization in the past three years. The polling evidence shows the degree to which Iraq has become a Republican war. And these latest numbers are also noteworthy in that they show that about one in four Republicans have now pretty much given up on the war.
All of which is to suggest that Democratic candidates will now probably be emboldened to take a stronger stance against the war. If principle doesn't win the day, at least the polling numbers are pretty clear what their base wants. Indeed, the polling numbers were pretty clear what Democrats and Independents wanted in 2004 - and the fact that they didn't receive the opposition to the war they were looking for from their standard-bearers is the main reason that they lost both the Presidency and did not pick up seats in either house of Congress.
(hat tip to Christy at FDL)