This is why Biden is running. Sanders/Trump hypothetical wasnt polled, but Sanders has never polled out of the margins of error the poll(s) have had.

"There's a silver lining for Biden, however. He leads the president by 9 points - 50 percent to 41 percent - in approximately 300 "swing counties" where the margin of victory in the 2016 presidential election was less than 10 points for either Trump or Democratic nominee Hillary Clinton.

Clinton won the cumulative vote in those counties four years ago, but only by a single percentage point.

"The race looks tight right now between Trump and the probable Democratic nominee. But as we learned in 2016, the outcome will be determined by the Electoral College rather than the national popular vote," said Patrick Murray, the director of the Monmouth University Polling Institute."