I think the answer to this question depends on how you interpret the rise of the "young anti-capitalist left" as it's termed in a recent, lengthy 538 report by Clare Malone. This group includes politicians like Ocasio-Cortez, organizers like Maurice Miitchell of the Working Families Party, organizations like the DSA and Justice Democrats and activist writers like Sean McElwee.
I take these folks seriously and think they perform an important role in raising the profile of some key left issues and generating energy and a sense of the possible around those issues. As Paul Krugman put it in Malone's article on the specific issue of the Green New Deal: "If the Green New Deal means that we’re going to try to rely on public investment in technologies and renewables and things that will make it easier for people to use less fossil fuel, that’s a pretty good start."
So all that's great. But it's important to keep in mind that the real left movement among the Democrats in party-wide and, for the median Democrat, that does not take them to the same place as this young anti-capitalist left.
Consensus Democratic leftism today rejects ‘business as usual’ and involves a sweeping indictment of the economic and political system for generating inequality and doing little to help ordinary people in the wake of the great financial crisis. Substantively, Democrats today – in particular aspirants for the 2020 Democratic Presidential nomination – are far more willing to entertain and endorse ‘big ideas’, such as going beyond the ACA, aka Obamacare (which is now vigorously defended) to ‘Medicare for all’, free college education, universal pre-kindergarten provision, vastly expanded infrastructure spending, including a Green New Deal and even a guaranteed jobs program. Taxing the rich is ‘in’ and worrying about the deficit is ‘out’.
Democrats are also highly unified on core social issues such as opposing racism, defending immigrants, promoting LGBT and gender equality and criminal justice reform. In short, the center of gravity of the Democratic party has decisively shifted from trying to assure voters of fiscal and social moderation, to forthrightly promising active government in a wide range of areas.
But this hardly means the Democrats are becoming a radical, anti-capitalist party. Far from it. As leftism goes, the current Democratic iteration is of a fairly modest variety, approaching, at most, mild European social democracy.
Nor is it the case that incumbents and moderates are being thrown out wholesale and replaced with candidates much farther to their left. Across the country, only two Democratic incumbents in the House lost primaries, and none in the Senate did. A Brookings study found that self-described “progressive Democrats” did well in primaries this election season but establishment Democrats actually did somewhat better. The same pattern obtained in the general election in November.Thus, the change in the party is less a leftward surge featuring new politicians with a radical agenda (though this is happening to some extent) and more a steady party-wide movement to the left.
So the Democratic party is moving to the left, but the young anti-capitalist left is only a part of that movement, rather than defining that movement. That's an important difference to keep in mind. The Democrats have been, and need to remain. a "big tent" party.