The Coming Conservative Majority
Many of the new Democrats in the House will hail from Red or Dark Purple states. In a presidential cycle (with Hillary Clinton as their presumed standard bearer), they'll have targets on their back from the get-go. They are also likely to be marginally more conservative than the Democratic mean. (IN '08 candidate Brad Ellsworth, who might represent a district Pres. Bush won by 12 points, is a good example.) These new Dems will pull their caucus to the right.
By "the right," read "social conservatism and foreign policy hawkishness." Democrats in tight races in Indiana, Ohio, Pennsylvania, and the rest of the swing districts are mostly running against voter malaise and Iraq war conduct, as opposed to the idea of the Iraq war. You're not hearing many Democrats rule out strikes on Iran or North Korea, and you are hearing many backpedal at Roadrunner-speed from citizenship-based immigration reform and gay marriage.