Oh it's much simpler and immediate than that: Trump wins and stops funding Ukraine, like he has already promised to dozens of times. That would instantly lead to the EU having to double their spending, which would certainly mean increased unrest within the EU and a whole lot of grudges towards the US.
And it wouldn't be the first time Trump specifically caused grudges. The US just giving up on the Iran deal has certainly already done the same. Not only was the EU also a signatory, but so were the UK, Germany and France individually. And then he moved the embassy to Jerusalem, which was also openly opposed by the three I've already mentioned. So now the EU has to second-guess every treaty the US signed, because apparently that signature means less than it used to mean.
But those two past actions would be nothing but a sentence in history books compared to at least a paragraph about the US cutting funding to Ukraine.