Lost the game
Banned
How do we get a dominant party in the United States? By dominance, I mean like that of the LDP in Japan, or the ANC in South Africa. What POD might me needed for this? How long would it last? How might it end?
You essentially had this in OTL between 1860 and 1932.
Kill off the GOP in the 1930s. There. You pretty much have it. Even if the Dems splinter, the New Deal Coalition is far stronger than the Progressives and the Dixiecrats.
anyone else?
Have things go very well for the Republicans after the 1960s: Popular Democrats keep getting shot, the Democratic party breaking up and factionalizing as the traditional elements of the New Deal coalition can no longer stand one another with the divisive topics of the times, and have Nixon succeed in forming a big tent Conservative party. He was relatively successful at that as it was, because look at the Republican party as it is today. Take all those Dixiecrats and working class people and bring them into the Republican party totally alongside the traditional white collar and wealthy Republican base, and leave the Democratic party without strong leaders and constantly divided and have no one left in the Democratic party be able to stand one another so they leave and form independent factions.