Here's how I see it. The leftmost member of the Republican Party moves over to the Democratic Party, thereby making it crystal clear that the Dems do not really play left to the Republicans' right, but rather that both are essentially centrist parties.
As long as this equation remains, there is no chance of the United States every rejoining the rest of the civilized world, which is social democratic. Now, that can happen.
It has hitherto been impossible for a third party to be anything more than an election spoiler. But attempts have hitherto been confined to dissatisfied members of one or the other of the main parties going off and creating an Independent Party, whether or not it is called such. Now, very differently, there is a real opportunity for the Progressive Caucus to secede from the Democratic Party to create a Social Democratic Party.
Part of the polarization is an attempt by the right to confuse an untutored public by conflating socialism with fascism, and by conveniently overlooking the fact that for 70 years, "socialism," unless specifically defined, was code for "communism."
Roosevelt's New Deal was bucking Stalinism. Supporters of Obama's New Deal have to be clear about what they want it to mean: Democratic Socialism.