A comment in the MetaFilter thread I referenced in my previous post caught my eye, and I started thinking (always a dangerous thing). The comment was:
I [think] the biggest question is: Why can't the party that won a popular majority in the last [presidential] election win a majority in Congress?
There were a few responses, but I don't think that any of them really hit the key issue.
In an effort to create more districts that could elect racial minority congressional representatives, a growing divide in the composition of congressional districts has occurred. This is pretty obvious when one considers the results of the 2000 elections. Bush won in a majority of congressional districts, even though Gore won the popular vote by half a million votes. The big difference is how the votes were distributed. Republicans tend to be more evenly spread over a large geographic area, while Democratic votes are concentrated in large cities, where there are enclaves with virtually no Republicans at all. The GOP has no analogue to overwhelmingly Democratic districts such as those found in New York City, Los Angeles, San Francisco, Chicago, and most other big cities. Even when a GOP challenger appears on the ballot, the Democratic incumbents receive over 90% of the vote. A "solidly Republican" district, on the other hand, is one where the GOP receives over 60% of the vote.
As a result, even though Gore received more votes than did Bush, Bush won (either through the electoral college as currently configured, or through a proportional system based on votes by congressional district). In fact, a Washington Post analysis conducted shortly after the resolution of the Florida fiasco indicated that Bush would have picked up an addition six or seven electoral college votes if the proportional system used by Maine and Nebraska were to be extended to the rest of the states. (I don't have a link to the article; I saw it on processed dead trees in late December 2000.)
This is interesting, because it indicated that Bush's support was more widely distributed than for the Republican congress that was elected at the same time. Additionally, except for Florida and little New Hampshire, all of the really close races were in states that went to GoreNew Mexico, Oregon, Wisconsin, and Iowa were decided by less than 15000 votes combined. The Democrats have more support, but the GOP has more broad-based support.