Not true, who the Democrats pick is important, they have to have a viable candidate who can campaign, otherwise it will wind up just like 2016, choosing the lesser of two evils, which would benefit Trump
And don't get ahead of the game, it will all come down to half dozen States, nationwide polls mean little which makes it easier for an incumbent, and Trump did win the majority of those States even if it was by the slimmest of majorities. The Democrat candidate has to win back the rust belt to win, and focusing on issues as the climate and Trump's legal problems regardless of how real they are aren't going to do it