Both Biden and Sanders would defeat Trump in November (RCP)

Monmouth Poll: Biden 48, Trump 45  (March 24)

Emerson Poll: Biden 53, Trump 47      (March 20)
Emerson Poll: Sanders 53, Trump 47   (March 20)

https://www.realclearpolitics.com/epolls/latest_polls/general_election/

I am not surprised, since most Americans believe that the country is going in the wrong direction.