We have a strong economy at the moment, and the fact that President Trump's approval isn't even 40% is pathetic. The incumbent advantage isn't as strong with such low approvals. People believe Trump is in a much stronger position than he actually is due to his shocking upset in 2016. Hillary Clinton was attacked by the GOP machine for DECADES. I have never seen such an effective political hit job performed before. (some deserving, some not).
If the Dems nominate a candidate with half the baggage that she had then Trump will once again be the underdog. Except this time, I don't believe he will emerge victorious.
The tax plan (if passed) will be a major liability among working class voters. People don't realize how hard it will be for Trump to actually run on a (nearly nonexistent) record. Many independents had the "wait and see" attitude about his candidacy. Obviously this will no longer be the case.