During FDR’s first New Deal, its largest opposition did not come from Republicans, but from Democrats further to the left. Father Coughlin, a Catholic Priest, mastered the radio as well as FDR and used this new medium to create an empire of loyal listeners. He complained that FDR had not done enough to curb the power of banks and wanted to see a government takeover of the Banking business.
An even louder voice was coming from Louisiana. Huey Long had campaigned for FDR, but when the President did not listen to his advice, he became his strongest opposition. Long created the “Every Man a King” campaign, where earnings would be capped and excess be dispersed.
I think we are seeing something similar right now in the Democratic Party, with the new left wing fighting against the mainstream traditional Democrats.