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.
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.
Polarization between the two major parties has for decades been mild. Since the election of President Obama it is becoming dramatic. This means that there are in effect three distinct political trends in this country: left, right and center. The now very small Republican Party having slid toward the extreme right, there has to be a party that is explicitly to the left of the Democratic Party, which occupies the middle ground--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."
Now is the time. Specter has opened the way.