I honestly think at this point it's obvious that Trump is being bribed by Putin.
The majority of Republicans in Congress support Ukraine and view Russia as an adversary. It is in the US national security interest to work with the EU to counter Russia, and in exchange ask the EU to help counter China and defend Taiwan (EU sanctions can help counter China's power).
Everything that Trump is doing with Putin goes against popular opinion of US citizens and of his own political party. There is no explanation other than Putin bribing Trump.