When we last took an overview of Senate races in December, Republicans appeared to be slight favorites to take control from Democrats, with a net gain of four to five seats representing the most likely outcome.

Since then, however, Republican fortunes have diminished somewhat because of problems with the quality of some candidates and key retirements. Although Republicans are most likely to gain seats on balance because Democrats have considerably more incumbents up for re-election, the question of whether the Republicans will win enough to gain control now appears to be closer to a tossup. In fact, the outcome may depend on who wins the presidential election, as well as whether an independent Senate candidate who is favored in Maine will caucus with the Democrats.