With the Senate knotted at 50-50 for each party, Republican control is only one seat away.
But recent fumbles by Republican candidates in New Hampshire, Georgia and Arizona have made finding those two winnable races more difficult.
Then came the accusation that Mr. Walker, a staunch opponent of abortion, paid for an ex-girlfriend's abortion, and the race turned upside-down.
Cook Political Report still rates the contest a tossup, but the new allegations have given Mr. Warnock an edge and put Mr. Walker on the defensive.
The Democrats’ best chance to pick up a Republican seat is in Pennsylvania , where Senator Patrick J. Toomey is retiring.