The GOP have a built in advantage in most districts due to gerrymandering. Here's Krugman on the subject, recapped at Bob Somerby's blog, to avoid the NYTimes missile defense shield:
http://www.dailyhowler.com/dh101606.shtmlKRUGMAN (10/13/06): Unless the Bush administration is keeping Osama bin Laden in a freezer somewhere, a majority of Americans will vote Democratic this year. If Congressional seats were allocated in proportion to popular votes, a Democratic House would be a done deal. But they aren't, and the way our electoral system works, combined with the way ethnic groups are distributed, still gives the Republicans some hope of holding on.
The key point is that African-Americans, who overwhelmingly vote Democratic, are highly concentrated in a few districts. This means that in close elections many Democratic votes are, as political analysts say, wasted—they simply add to huge majorities in a small number of districts, while the more widely spread Republican vote allows the G.O.P. to win by narrower margins in a larger number of districts.
(snip)
KRUGMAN: My back-of-the-envelope calculations suggest that because of this ''geographic gerrymander,'' even a substantial turnaround in total Congressional votes...would leave the House narrowly in Republican hands. It looks as if the Democrats need as much as a seven-point lead in the overall vote to take control.Also, the Republicans have a much, much better organized GOTV program. They spend tons of money identifying voters and getting them to the polls - - they've spent tons more money on this for many more years than we have.
And they focus on it. They were surprised when Gore won the popular vote in 2000, so they spent a couple million studying his GOTV program. They then used those ideas developed by Gore's team to help them out mobilize us in 2002.
The DNC likes to "brag" at the end of the 2000 campaign Gore got 51 million votes and the DNC didn't know who a single one of them was.