Rep. Debbie Wasserman Schultz (D-Fla.) ruled out a 2016 Senate bid on CNN Tuesday.
The GOP group is launching robocalls against Reid over an anti-trafficking bill.
The former Florida governor had been eyeing a Senate run in 2016.
The House Democrat says bids by Van Hollen and Edwards haven't deterred him.
Survey from Democratic-leaning Public Policy Polling gives the Democrat an edge in rematch.
House Dem leaders with Md. ties won't follow in Reid's footsteps and wade into the primary.
The endorsement puts further pressure on Democrat P.G. Sittenfeld to back out of the race.
Progressive favorite's decision sets up primary fight with Rep. Chris Van Hollen and others.
GOP congressman would consider run if Rubio tries for White House.
Alaskan had the lowest 2014 score of any GOP senator, and the group could primary her.