Green’s Dictionary of Slang

(US) a sudden political change of policy.

1880		N.Y. World 22 Nov. 5/1: Mr. Skinner’s apparent flop on the railroad question is injuring his chances in the Speakership struggle.
1904		Springfield (MA) Weekly Republican 7 Oct. 2: That a flop by the most militant of the unionists is under contemplation has been denied.
1911		H.S. Harrison Queed 230: So ran the editorial, which was offensively headed ‘West’s Fatal Flop’.
1929		Collier’s 5 Jan. 41/1: It was basically a ‘flop’.