As the Korean War drags on, Truman's popularity at home plummets. The Republicans nominate the former general and war hero Douglas MacArthur, who had been relieved of command by Truman for insubordination. MacArthur wins in a landslide victory.
Let's review:
MacArthur has no problem using nukes; in fact, he advocated using nukes against China, though not necessarily on civilian targets.
MacArthur wanted to drive across the Yalu into China.
The American public is terrified of Communists, as China has "fallen" to Communists, the Warsaw Pact has been formed, the Soviets have the Bomb years early, and McCarthy is in power.
What happens?