I'd consider these to be essential conditions for such a scenario
1. Killing Stalin off in the latter days of WW2, with leadership of the CPSU going to Beria. Somehow, you need enduring pragmatic political liberalization and a foreign policy that can't even be remotely construed as threatening to global capitalism.
2. Have Henry Wallace somehow manage to stay on the ticket in 44, and become president following FDR's death. Wallace then needs to successfully manage the transition to a peacetime economy. If he can keep organized labor happy, and not heavy-handedly crush strikes like Truman did, there is a decent chance that his party can keep control of the Congress in 46.
3. Wallace needs to be a transformative figure in the Democratic Party. That means winning re-election in 48, and defining the national foreign policy agenda for the decade to come. He has to sell peaceful friendship and coexistence with the Soviet Union to the liberal managerial elite in government, and to somehow defang the business community that itched for independence and pathologically feared socialism in the era of technocratic corporatism.