Currently the Democrats hold 51 seats in the U.S. Senate. Three Republican senators are retiring — in Colorado, New Mexico and Virginia — one, Ted Stevens of Alaska, has been convicted— so it appears all four could go to Democrats.
In Minnesota, New Hampshire, North Carolina and Oregon, the incumbent Republicans are seriously threatened.
Three other races, Georgia, Kentucky and Mississippi, are competitive. Still seems a longshot for the Democrats to get a filibuster-proof Senate.