Gallup has just released its comprehensive analysis of post-election polling data, and it shows that a key reason Dems won was that in addition to enjoying solid base support, they also made important gains among whites, married voters and rural voters -- traditionally GOP-dominated groups -- as well as among independents. Says Gallup:

An analysis of Gallup's final pre-election poll data shows that the Democratic victory in Tuesday's House elections was because of a rising Democratic tide that lifted support in almost all key subgroups. In addition to solid support from their core constituent groups such as liberals, nonwhites, women, urban residents, and older Americans, Democrats also owe a significant debt to independent voters, who tilted strongly in their direction. Whites and those who are married -- groups that usually favor the Republican Party -- were evenly divided in their vote. Democrats did better among rural voters, a change from previous voting patterns.

Opposition to the Iraq war appears to have helped the Democratic cause. Although supporters and opponents of the war voted about equally for the Republican and Democratic parties, respectively, the fact that opponents outnumbered supporters made Iraq a net-plus for Democrats. In general, it appears that Democratic efforts to get out the vote played a significant part in the win, because those who were contacted by Democrats and urged to vote for Democratic candidates strongly supported Democrats, even if they were also contacted by Republicans.

Key stats: Nearly half of whites and married voters voted Dem this year, more than in previous years. Even more interesting, some 55% of both rural and urban voters chose Dems -- whereas in past years rural voters were significantly less likely to vote Dem than urban voters were.

Gallup's full analysis here.