Trump's criticism of EU and UK leaders, his criticism of NATO, his private meetings with Putin, his questioning of US intelligence agencies (with his equating of Dan Coats and Putin, and his saying Russia's offer to help with the Mueller investigation was an incredible offer
Trump seems to believe that anything short of hugging Putin and giving him a massage could not be seen as collusion with Russia, or as providing "aid and comfort" to Putin.
Then, of course, there are the myriad money trails, from his early ties with Sater to the Butina links. And Trump's efforts, during his presidential campaign, to get a Moscow Trump Hotel deal.