Poor comparison not based on actual numbers.
Dems have been able to capture more than 50% of the vote statewide multiple times, multiple offices over the past decade. Only Sherrod Brown can win a statewide race in Ohio.
He doesn't need his base to grow. He needs to win WI, PA, and Michigan. He can literally lose every republican in NY and California and it wouldn't change the election.
Its voter turnout. A lot of people who sat out in 2016 are pissed and will come in a record turnout in 2020. MI, WI, MN, and PA all voted out their GOP people in 2018 Midterms. And he is losing massive ground in the Midwest in a record pace. Voter turnout is everything, not to mention the Electoral College being turned down in more states.