E: 1.38, S: -0.51
The Democrats have a couple of "structural" advantages going into the 2010 Congressional election which prove crucial.
Thinks of these as akin to "home field" advantage in a football game.
In the Senate, more seats held by Republicans will be up than those held by Democrats (this is the consequence of 2004 being a good year for Republicans).
In the House, we are at the end of apportionment cycle and seats held by Democrats in the House have significantly smaller populations (for the most part) than those held by Republicans. So, even though the relative aggregate ratio of Democrat to Republican votes may significantly improve for the Republicans, a corresponding increase in members of the House will not likely occur.