Pelosi’s specific concern is that if the 2020 election produces a very narrow victory for Democrats, Trump will reject the outcome by claiming that it was rigged, perhaps by regurgitating vacuous claims about Democratic voter fraud. 

…Pelosi said only a decisive, overwhelming victory will convince him to step aside.

Pelosi said Trump might not leave office if he loses in 2020. He once said himself he might not. – ThinkProgress

hmmmm