The Dems. actively oppose the GOP politically, true, but it is a power struggle not not necessarily based on the will of the voters. If a Dem win's the White house in 2 years, we will have the mirror image of what he had with the GOP controlling the Legislative and Executive branches of government. and the Dems won't oppose themselves. Neither party has shown a willingness to respect public opinion except when it comes to winning the election at hand.
Yet elections are where it's at for them. Don't think for a moment the DNC or RNC failed to notice the lesson of the last election (really, 2004, as well): you alienate your base at your peril. Right now, the Dem's base is dovish. Indeed, you could see Congressional Dems creaking under the strain of the pretense of supporting the war these last few years -- it really wasn't in their nature, but they went along to get along. With no real need to support the war, why would they?
But let's say for a moment that the Dems will do what they want, voters be damned. Why
would they drum up a new war or intensify the old one? It's simply not who they are. The current crop of Dems was formed from the anti-Vietnam Left of the late 60s / early 70s. Vietnam is the lens through which they view everything.
One could argue that the current GOP leadership is using the war to increase political/state power while at the same time giving limited government lip service. Ok, fine. But historically (well, any living history) the Dems don't bother with that. They
explicitly venerate the State, and openly clamor for increased state power. They don't need the facade of an unpopular war. Increasing state power is their candid platform.
My point is that the two parties have roles, approaches, strategies that they're comfortable with, and they tend to not use the other party's persona to any great extent.