Not e-voting. It's a combination of (1) geography (2) opportunity and (3) winner-take-all.
No, not the voting machines.
The combination of single-member, winner-take-all districts and our partisan geography (where Republicans have the advantage in terms of surface area) tilts House elections in favor of the GOP.
The constitutionally mandated malapportionment of Senate districts exacerbates the tilt in that Chamber.
Some examples of wrong-winner Congressional elections:
1) 2004 U.S. Senate: Democrats win 51% of votes, yet Republicans take 55 seats with only 45% of votes.
2) 2004 Connecticut House delegation: Democrats win 55% majority of votes statewide. Republicans just over 44%. Republicans get 3 seats; Dems get 2.
Democratic voters are more likely to be geographically concentrated in urban and suburban areas; they live in naturally `packed' districts. By contrast, Republican voters are more efficiently distributed for the purpose of winning congressional seats. Thus, George Bush could lose the national popular vote in 2000, but carry 47 more of the 2002 congressional districts. That pattern of Republican advantage in was even more pronounced in the 1970s, when far greater ticket-slitting concealed its impact.
As another measure, the average margin of victory in 2004 in a Democratic-held district was 45% points - in contrast to 37% points for winning Republicans.
In all probability, Democrats need to bust the 53% mark to claim the House.
There's a little more to the story than geography and stupid, old electoral systems. Assume the Dems have a 1994 of their own - that is, they snag 54% nationwide. There are simply just fewer opportunities for them to win seats.
In 1994, there were 52 open seats. 37 of them were in districts of Republican partisanship. The GOP won 32. Today there are only 30 open seats, and only 11 of them represent districts of Democratic partisanship. (More...)
Make no mistake; the Democrats face an uphill battle, even as the majority of voters think we're going in the wrong direction.
Equally interesting questions are how long the Dems could hold such a majority, what the pickup of seats in R-partisanship districts means for policy and caucus politics, and what that dependence on Republican voters means for the '08 nominee.