I'm of the Michael Brooks school of thought on this, where I don't actually think 45 is as tough to beat as people believe tbh (assuming things don't take a major turn like a military conflict is provoked, scandals, etc). That's to say, I look at the 2018 results as an indicator that the Trump cult of personality will see lead not just to his blind supporters but an abundance of people that do the opposite and vote against him regardless of who the Dem candidate is (we had a 50 year high in turnout for a midterm election). I do admit though, this is partially because I'm suspicious of the whole "electability" concept as something the centrists use to push their own preferred candidates. I think anyone who can give a clear, firm message about what direction they will move this country in will get a lot of love just off the relative comparison to Trump's 4 years of incoherence which will be even clearer this time around than last time. Everyone knows my leaning is progressive but I'd expect even cats I'm partially or not at all fond of to do well against him too...Beto, Pete and Kamala for example. They're not my favorites philosophically, but they probably beat Trump imho.
Admittedly all of that is pure speculation though.