If the Democrats re-gain control of the House, which pundits and prognosticators say they will, it will do so without the old Confederacy as a base of support. This is a big deal, and signifies a shift in the politics of the South and the Northeast.
But it doesn't necessarily mean that the Democratic Party itself will become more Liberal. Many of the new seats to be gained are by definition in Republican held areas. Sure, some are in blue states or areas Kerry or Gore won, but a majority will only come from Republican areas (that is, Bush districts and some in Bush states).
For example, Indiana seems to be fruitfull territory this year for House seats. But Indiana has supported Republicans for President for decades, and they just elected a Republican Governor. Granted, the Republican Governor's unpopular decisions are part of the reason Democrats look good in that state this year, but these candidates are not liberals by any means.
There will be a lot more new Melissa Beans in Congress on January 2007 than there will be Chris Van Hollen's because of where Democrats will be sucessful. To me, it looks like Connecticut will not be a good state this year. All of the GOP incumbents knew it was going to be a tough year, and were prepared. Some may still lose, but I doubt all of them will. And Joe Lieberman's continued presence really hurts Democratic challengers, as Joe's quasi-Republicanism becomes more pronounced with each day.
Upstate New York will yield a few, but again these are in conservative areas...even if Clinton will carry them twice and Spitzer will run circles around the GOP. The region still elects GOP state Senators and voted for Bush in 2004. Ohio's gains will come from very GOP regions, as will any California gains (1-2 might materialize).
By contrast, Penn. will be a good state for Democrats this year, so more progressive Dems will come from there. There are a handful of seats that if they flip will like Chis Van Hollen's stay flipped: Northrup's in KY, almost all of the CT districts, the Philly burbs, Wilson's in NM...but the rest of the gains will be challenged very hard in 2008, when turn out will be higher and the GOP will have NOTBush as their presidential candidate (most likely McCain). So all of the class of 2006 will play it safe and not vote especially liberal on social isssues (save stem cells).
The same story will be true for the Senate as well. A Senator Ford, Webb, McCaskill, Casey, and probabbly Tester would vote fairly moderately and make pains to disagree with their liberal collegues when possible.
Bottom line: the New Democrat caucus in both the House and the Senate will not die out. While the Democratic majority will not be beholden to the South in the House, it will in the Senate. But again, either way, there will be more moderates in both chambers after 2006, not less. This will not make the would be Democratic majorities more liberal. If anything, House Democrats as a whole will be less liberal.