Well, there's Trump's long history of attempting to break into the Russian real estate business (which has been very difficult for him and which he has been attempting for many years), and his Moscow Trump Tower project. He also has repeatedly claimed he has no financial interest in Russia. Honestly it's fine for a president to have interest in opening a hotel in Russia, but why is he lying about it.
"Russia has never tried to use leverage over me," he tweeted in January 2017. "I HAVE NOTHING TO DO WITH RUSSIA - NO DEALS, NO LOANS, NO NOTHING!" (from this article on businessinsider:
https://www.businessinsider.com/trump-russia-business-financial-ties-2018-11)
Yet this is untrue. It's weird he would deny his long history (up until current) of business dealings with Russia:
Business Projects of Donald Trump in Russia Wikipedia page.
Additionally, after declaring bankruptcy several times in the US a couple of decades ago, US banks would no longer lend to Trump and he found those who would still buy into him, which happened to be wealthy Russians:
I bolded the parts that are most important IMO. This paints a picture of a guy bailed out by powerful Russians, with ties to the Russian mafia. We know that his team met with Russians at least 14 times during his campaign. Now I'm not saying there is anything wrong or illegal about having business dealings with Russians, obviously there isn't. What I am saying is that he is in debt to powerful Russians and he has business interests in Russia, and he lied about that on Twitter. And it's not too big a leap of logic to think there is a good possibility that said Russians could apply leverage and use him as a pawn in interfering with foreign affairs... he is the president after all.
As for Uranium One/DNC hacking, to me that's whataboutism. Yes, that shit is fucked up. So is this. My point is, this is fucked up.