Virginia

Five-term Senator John Warner, the top Republican on the Senate Foreign Relations Committee, announced in August he will be retiring at the end of this term. Almost immediately former Governor Mark Warner, a Democrat and no relation to the retiring moderate Republican, announced his intention to run for the seat. Warner will be tough to beat given his recent popular stint as governor — a term-limited position in Virginia. Two big GOP names have expressed interest in taking him on: another former governor (and until recently a long-shot presidential candidate) Jim Gilmore and Congressman Tom Davis, a moderate who represents the DC suburbs. "It's not over by a long shot but Mark Warner is a very, very strong favorite," says Charlie Cook, editor of the non-partisan Cook Political Report, which tracks congressional races.

— Jay Newton-Small/Washington