It's an important question. If you listen to the offical position of the DNC it is that anyone who votes Republican is either evil or stupid. This could be why the Democrats have lost the last two Presidential polls and every Congressional election since 1992. But put them on the spot, when actual votes are at stake, and they rapidly back away from the official position.
We all remember the Florida débacle of 2000, when Al Gore declared that anyone too stupid to vote in a clear and discernable way, probably meant to vote for him. Could be true, but tough.
Now in Ohio Republican Secretary of State (and gubernatorial candidate), Ken Blackwell, wants to issue voters with stickers that say "Change the world: vote your conscience". Democrats complain that this is a subtle steer towards voting Republican. The word 'conscience' they claim equals Republican. I am not sure that is true. I think there are many people who, guided by their conscience, choose to vote Democrat. Why do Democrats no longer believe this?
They want to stick with the former slogan "I voted today". The problem is that the old slogan does nothing to promote turnout. It is just a sanctimonious claim of moral superiority. Blackwell's slogan rather implies "I voted, and you should too".
Democrats used to think higher turnout was in their interests. I guess 2004 disillusioned them.