To me, the big question in all this is not if Democrats can win the election from Trump, it's what is going to happen when he refuses to honor the results and refuses to leave. Everything he has done so far has pointed to him staying in power no matter what the rules are. Rules do not apply to him. 

From the attached video: 'My question to all Democratic candidates, what is the plan if you win, and the next day Trump claims he's voiding the election because of some 'irregularities he's hearing about'? What do you do when his base 22,000 strong, marches on Washington? What do we do if he doesn't go? Do you really think if Democrats win by a landslide, Republicans will suddenly get rational about math and facts? You think the same Senate that refuses to hear witnesses and acquitted Trump of abuse of power, is going to stop Trump when he calls the election rigged? Who is going to stand up to him, the Attorney General who works for him? The Supreme Court Justices that he appointed? Who is going to make him leave the office when he refuses to leave?'
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Trump: 'President for life. I think it’s great. Maybe we’ll have to give that a shot some day.” Trump: 'My people may demand that I stay longer.' Trump: 'I might remain in the Oval Office at least for 10 or 14 years.” Trump shared a tweet: “Trump should have 2 yrs added to his 1st term as pay back for time stolen by this corrupt failed coup.”