That's childish Republican demagoguery. Republican politicians supported these wars at least as enthusiastically as Democratic ones. In fact, in 1964, LBJ ran as a peacenik, insinuating that Barry Goldwater was a reckless warmonger. But when he became President, he became just like Barry Goldwater in warmongering.
You are only partially accurate. Prior to Pearl Harbor Republicans were isolationist, and very anti-war (my father, in his youth, being one of them). FDR had to hide his pro-war desires by running on a "no war" platform, while promoting the US mission as one of being merely an arsenal for democracy. After the war, the Soviet Empire and the Truman doctrine set a new direction in American foreign policy, and the cold war began.
By Korea there was not a big difference between the two parties, the Taft isolationist wing losing to the "moderate" Eisenhower. By Vietnam, the big difference was that Goldwater's position was that no war should be fought without the objective of total victory, and that "police actions" of indefinite containment, like that of Korea, were unacceptable. Johnson foolishly believed he could somehow "win" by defensive containment, without declaring war on North Vietnam and using all means at his disposal.