A good thing would be for the Northern Democrats to be taken over by the Populists instead of vice versa. Avoid the splintering of the left (indeed difficult, but if the class situation in America gets worse I could see a figure like Bryan leading to the unification of small dogmatic parties), have DeLeon strike up a friendship with Bryan (or whoever) and his ideas become dominant in the left parties. The Socialist Labor Party then takes over the Populist Party from within.
This leaves the Democrats as a Southern Party, Republicans as a business party and Populists as a progressive worker's party. In such a situation, a *Great Depression happening leads to a Populist Government. And the sheer size and influence of America would then inspire Marxist-DeLeon movements elsewhere.
Is Marxism-DeLeonism going to be a part of your TL?
That's a start, I suppose. The problem, though, is that the Populists as they were IOTL were too regionalist and too tied to the economic concerns of the rural, western states. This alienated them from the industrial states in the North, which meant they didn't make much of an impact there.
However, if they can avoid being taken over by the Democrats, I could see a more far-sighted Populist leader making a strategic alliance with urban, often immigrant, worker groups. Overcoming the nativism in the Populists would be hard, but do-able.
So I suppose you could have the SLP develop independently, perhaps taking over the AF of L's leadership role in the labor movement, and then working as a sort of federal alliance with the Populists.
The two parties would probably remain distinct for a time, but would heavily influence each other before merger. If DeLeon plays his cards right, he could become the American LaSalle. Populists would become Canadian CCF style prairie socialists, espousing some version of syndicalism as well.
And yeah, Marxism-DeLeonism will be making a splash in my TL, with post-revolution leftists rediscovering DeLeon as they grapple with reconciling competing currents of anarcho-syndicalism, left communism and Marxism-Leninism.