Here's the thing: Not only have they not won anything yet, but things are too close in too many races to start celebrating. This is the problem--they haven't closed the deal.
Republicans know they have to close the deal (which is their problem with this whole Iraq thing, their rhetoric is locked into their need to finish the job). They're going to come on strong, and maybe they'll still fail with the Foley thing, the Woodward book, and whatever other things happening, but it will be them failing instead of the Democrats winning.
I recognize what the Democrats are doing because I used to do it too. I'd assume that my grades in a class would come out alright--I was smart, I knew the material, and assumed I'd carry through. But I didn't close the deal--I'd miss some homework, I wouldn't lock down the material for the exams, and I would kind of muddle through. Now, I didn't do that badly, but I probably could have done better. Nowadays, if I want to be successful, I don't take anything for granted--I try and make sure every aspect of a project or plan is good, I engage in follow through, and I generally try and put everything in order so my success is virtually guaranteed.
The Democrats are the bright kid who just assumes things will work out to their advantage without finishing the job. They need to be relentless the next few weeks. Campaining, advertising, never letting the pressure down on the Republicans. The Republicans have been stacking the desk and being relentless for over 2 decades, and it's only gotten them the slimmest of majorities, which they've then continued to use as a blunt instrument to get what they want. Whenever the Dems even begin to fight it falls apart. Imagine what would happen if the Dems actually did what the Republicans did. It's tempting to say "Even if they only did half as well as the Republicans they would cruise to easy victory," and it may be right, but if you compromise yourself that way you're setting yourself up for failure.
So here's to hoping the Democrats can run through the finish line instead of stopping at it.