Disclaimer. Don't rely on these old notes in lieu of reading the literature, but they can jog your memory. As a grad student long ago, my peers and I collaborated to write and exchange summaries of political science research. I posted them to a wiki-style website. "Wikisum" is now dead but archived here. I cannot vouch for these notes' accuracy, nor can I say who wrote them.
Jacobson. 2005. Polarized politics and the 2004 Congressional and presidential elections. Political Science Quarterly 120: 199-218.
If you're looking at the 2006 elections, the Republicans are in good shape at this point. In 2006, the Republicans will probably try to mobilize along party lines the way they did in 2002 and 2004. As long as people vote on party lines, Republicans win (because Republicans are more spread out than Democrats).
Not a theory so much as an interpretation of the upcoming elections.
Research by the same authors
Research on similar subjects