I agree with the other posters in saying that he would not win the Democratic Primary in 2020. He is no Bernie Sanders even if it may seem like it. As for Maps if he somehow wins the Democratic Primarys:
Donald J. Trump - 48.31%Angus S. King - 48.96%Id doubt he would excite the Liberal base and thus there would be a slightly depressed Liberal vote for King and also Depressed Minority vote as id doubt he would excite them too. Trump makes gains in the rural areas breaking upwards of 60% in new states and also taking advantage of the Kings weaknesses and snatching Nevada due to lower Hispanic Turnout and depressed liberal turnout in Clark County and also keeping Colorado and Virginia closer. The only areas King may do better then clinton in may be in Rust Belt and the Northeast. The Rust belt due to the fact that he is not Clinton and also is supportive of some tariff deals and the Northeast and specifically Upper New England in New Hampshire and Maine due the fact that Maine is his Home state.