In the early 1920s, American "progressives", the first "dreamers", fell in love with the romantic concept of a "worker's revolution", as done by the Bolsheviks in 1917. Woodrow Wilson was an early supporter.
While the Bolsheviks may have had good intentions to create a pure communal society, unknown to them, one of their early leaders, Vladimir Lenin, was an opportunist. He let them do the heavy lifting of overthrowing the Czar, then started murdering selected leaders, and working his way into leadership. Eventually all the original members of the Politburo were murdered or driven into exile.
American progressives seem to have missed all of that, as they continued to believe, for decades, that Communism was a mild societal structure based around sharing. Many still do.
In 1936, civil war broke out in Spain, an early attempt by Stalin to convert Europe to Communism. In America this was portrayed as a "worker's revolt", and thousands of Americans flocked to Spain to fight on the side of the Communists. There were so many that they were formed into a unit called the "American Legion". They were armed with thousands of Mosin-Nagant rifles supplied by Stalin.
After WWII, American progressives became "peaceniks", and insisted that the USSR was a peaceful nation, and that we were the bullies (sound familiar?). Sen. Joseph McCarthy led a House committee (HUAC) in a search for Communists in govt and media. He was half-right. There were very few card-carrying Communists (Alger Hiss notwithstanding), but there were many progressives very sympathetic to them. This came to a head in 1953, when Julius and Ethel Rosenberg were executed for treason for turning over to Russian spies technical data on the atomic bomb. Despite overwhelming evidence, some progressives still maintain that they were framed.
American progressives/liberals/democrats/Marxists/etc have a long history of either supporting or being sympathetic to Communism, which they see as a harmless society based on communal sharing of resources. This is complicated by the fact that some policies of FDR leaned heavily on Marxist ideals (Social Security, Medicare, Works Progress Administration), as did the Income Tax and Obamacare.
Most Americans today were not yet born when FDR started us down this road, so they accept these ideas as a normal part of a compassionate society, like open borders.