1948: No third-party candidates

[President Johnson]

How would 1948 have gone without Henry Wallace and Strom Thurmond? Would Truman's victory be more comfortable in such a scenario?

Probably Truman would have won New York (Wallace took enough votes to give a narrow majority to Dewey) and some states of the Deep South, although the latter were strongly opposed to his civil rights policies.

[🐒Gods of Prosperity🔱🐲💸]

fairly straightforward Truman victory. 

Without Wallace, Truman gains New York, but Dewey keeps his other states (a few under 50% still thanks to Socialist and Prohibition candidates).

The Democrats win all Southern states, though Virginia is close, and in the Deep South some electors for the Democratic party are understood to be unpledged.

Truman/Barkley  53.2%  389
Dewey/Warren    45.7%   142
others                  1.1%

[Unconditional Surrender Truman]

It would be a Truman landslide. Keep in mind that, without Wallace and Thurmond, there would have been less of a perception that Dewey was the inevitable victor. David McCullough argues that Dewey's seeming inevitability kept many Democrats at home in the east coast states, so I could see NY, NJ, PA, MD, and DE flipping, in addition to the South going reluctantly for Truman.

Truman (D): 448
Dewey (R): 83

[Sumner 1868]

The only way that the Dixiecrats don't form is if the civil rights platform fails and the military maintains Jim Crow, which would cost Truman the black support that proved crucial in swing states, and probably mean his defeat.

[Sir Mohamed]

I don't see 1948 without a Dixiecrat ticket. A scenario without a progressive ticket would be possible, but the Southern racists strongly opposed Truman's racial policies and he, although I admire this president, wasn't that kind of figure like FDR who hold the party factions together.

Even without an official convention, some southern states would have voted for write-in ticket.

Harry S. Truman (inc.)/Alben Barkley: 388 EV. (52.5%)
Tom Dewey/Earl Warren: 115 EV. (45%)
Unpledged electors (Strom Thurmond): 28 EV. (2%)

[Fuzzy Says: "Abolish NPR!"]

I don't see 1948 without a Dixiecrat ticket. A scenario without a progressive ticket would be possible, but the Southern racists strongly opposed Truman's racial policies and he, although I admire this president, wasn't that kind of figure like FDR who hold the party factions together.

Even without an official convention, some southern states would have voted for write-in ticket.

Harry S. Truman (inc.)/Alben Barkley: 388 EV. (52.5%)
Tom Dewey/Earl Warren: 115 EV. (45%)
Unpledged electors (Strom Thurmond): 28 EV. (2%)

The only states Thurmond and Wright carried were states in which the Democratic electors were pledged to Thurmond and Wright; they carried NO states where they were a "third party" slate.

The makings of a shift to the GOP by Southerners was in process in 1948, but Dewey was a liberal Republican, and not the kind of guy establishment Southerners like Harry Byrd would want his state to get in the habit of supporting.