Congressional Quarterly (CQ) is a major publisher of databases and publications related to U.S. elections and governement. This freely available website covers congressional politics in depth. Type your state name in the search box to find articles concerning your state. As the Fall elections gets closer, coverage expand and likely will include a map of all U.S. congressional districts with substantial info about the current and previous races, as well as CQ's ratings on the likelihood of political change in the district.
This site provides listings for congressional races across the nation. Although the main purpose of this site is to track the flow of money supporting candidates, it also provides valuable info about the candidates and previous elections.