He's a social democrat offering a less socialist version of the NHS. He is outpolling Trump in the key states he needs to win back, i.e. in the mid-West, Pennsylvania and even Texas is looking close. The Republicans call any Democrat a communist. All he is doing is offering policies that are very popular with a majority of the American people and if you'd seen any of his events with FOX and apparently conservative audiences he has done very well indeed.
All of this of course ignores an even more basic point. The Democrats got the ultimate centrist candidate that they wanted in 2016, and got annihilated in the electoral college. Their centrists have also overseen the Republicans gaining majorities in the Senate, of state governorships and of state legislatures. And who beat Clinton? A celebrity who bragged about molesting women but who also sounded like an economic populist.