Winning? Get your head out of the sand. "Stumbling" is a more appropriate verb to use in your sentence. Republicans will not even want him as their nominee for 2020. And the party will by then be half ripped-apart due to Trump's existence and manner in Washington. Also, the next Congressional elections are in just 16 months. Dozens of Republicans in tightly-fought constituencies are by now realizing that things are going "very wrong" for them at this point. Trump won't lose his most rabid supporters, but the party will split.