I don't know. Japan was an unstable place post-war, and if the US mismanaged the occupation (which is not an unrealistic possibility) then there's always a chance. In the 1949 parliamentary elections IOTL, the Japanese Communist Party took 10% of the vote. That share could potentially be higher.
Cheers,
Ganesha
No. The Japanese Communists had the same chances to win democratically than the East European Communist parties: 0 %
The only way possible is a Soviet occupation, but the butterflies are so big and so old that frankly ...