In 1935, there was a communist revolt in Brazil that was quickly suppressed. What if it succeeded? FDR is following his good neighbor policy towards Latin America and because the US is isolationist, I doubt he would interfere.
How would communist Brazil affect the Cold War later on?
Sorry, but there is simply now way it could have succeeded. First, the rebells simply didn't have any important support among the population. It was rather a military rebbellion inspired by communists than a proper communist rebellion, given the fact that all the conspirators were from the Army. It had nothing to do with organized workers. The majority of them were simple soldiers who knew nothing about communism, they only believed it was something that would be good to Brazil. If the officers that convinced them to join the fight had told them that Fascism was the solution for the nation's problems they would join the fight too. They fought more due to loyalties and dislike of their commanders rather than ideology. It also meant that when the rebells asked some of their fellow officers to join the movement many of them decided that they needed to inform their commanders about it or even try to convince the commanders to join the rebellion.
Also, the government was well aware of the insurrection, and only in Natal, the lest important of the three cities where they rebelled (Natal, Recife and Rio de Janeiro) the movement managed to occupy a public building other than their own headquarters, and they simply fled and gave up when they saw a loyal company arriving the to city.
Finally, it only happened because Luis Carlos Prestes sent wrong information to Moscow, informing that support for communism was widespread among the Brazilian population. The contacts between him and the Politburo were known by the British, who informed Vargas that a revolt was likely to happen. The rebells simply had no chance.
Unfortunately I couldn't find any good online source about it in English, but there is this good work of Frank D. Mccann: "Soldiers of the Pátria: A History of the Brazilian Army, 1889-1937" (Stanford University Press. ISBN 0804732221). He gives a lot of interesting information about the movement and why it was doomed from the start.