Green’s Dictionary of Slang

one who slanders, talks maliciously about somebody.

1876		Wkly Marysville Trib. (OH) 4 Oct. 2/4: The democratic mud-slingers of Ohio are [...] concentrating all their energies to bespatter this gentleman with their filth.
1889		Abilene Wkly Reflector (KS) 24 Jan. 1/3: A Sorehead Mud-Slinger. It was necessary that the mud-slingers should get someone to do their dirty work.
1896		Omaha Daily Bee (NE) 3 Oct. 8/4: [headline] Dr Hirsch Arraigns Mud-Slingers.
1906		Albuquerque Eve. Citizen (NM) 22 Oct. 1/3: The Morning Mudslinger continues its daily outpour of filth and falsehood.
1911		Coconino Sun (Flagstaff, AZ) 24 Nov. 4/3: The mud-slinger has to muss himself all up in order to cast his dirt at other fellows.
1915		M.G. Hayden ‘Terms Of Disparagement’ in DN IV:iii 207: mud-slinger, a slanderer. ‘Don’t be a mud-slinger. You might soil your hands.’.
1920		Logan Republican (UT) 8 May 2/2: Like most mudslingers his yelps at the come back are piercing.
1926		Wood & Goddard Dict. Amer. Sl.
1931		Derry Jrnl 2 Mar. 4/5: His denunciation of the slanderer and mud-slinger.
1943		Wash. Court House Record-Herald (OH) 26 Oct. 1/2: [headline] President [...] Gives Stern Denial To Charges By ‘Mid-Slingers’.
1948		Worker (Brisbane) 27 Sept. 4/4: Mr Casey is doing his best — or his worst — to organise the mud-slingers.
1952		Big Spring Wkly Herald (TX) 31 Oct. 8/6: ‘You’re just a low down mudslinger’.
1962		Troy Record (NY) 8 May 14/1: Time after time the voters have rebuked the mud-slingers.
1973		Van Nuys News (CA) 21 Sept. 2/4: [headline] Mudslingers Aim Barrage of Allegations at Agnew.
1983		Des Moines Register (IA) 6 Oct. 23/6: ‘He’s a mud-slinger. It’s easy to [...] criticize when you’ve never done anything .
1990		Florida Today (Cocoa, FL) 24 Apr. 9/2: We’ve hired mud-slingers — the best in the business.
2007		Chicago Trib. 24 Jan. 15/3: Hilary Clinton’s and Barack Obama’s quests for the White House have already had to endure the mudslingers.
2014		Clarke Co. Trib. (Quitman, MS) 22 May A10/1: Mudslingers and Bullies. I keep getting a card [...] from a poliical candidate who does nothing but sling mud.