Politics is dynamic. So it’s not that McGrath absolutely can’t win. But she probably needs at least one of three things to happen:
1. Trump to become much more unpopular than he is now so that he would be weaker in Kentucky, too.
2. Conservative-leaning voters to put aside their general partisan preferences because they are tired of McConnell, even as they remain aligned with Trump.
3. McGrath’s particular appeal as a former Marine to get conservative-leaning voters to support her, even as they remain aligned with Trump.
None of that is impossible. But none of it is particularly likely, either.