Okay, I came back ;-). Hopefully this is the last time. I hate talking about this stuff, but it's important, and I really appreciate your OP.
A few of the most important tree trunks are:
* Unilateral softening of sanctions against Russia, plus recent refusal to enforce the sanctions that are left, combined with lots of reasons to think Russian money ended up in Trump's pocket from 2012 onward, combined with refusal to release tax returns.
* Lots of money laundering connections between Trump and Russian oligarchs, which are the same as their government.
* Every top official who's involved with the Trump-Russia organization has either been fired or come under public attack by Trump. And they're (almost) all Republicans.
* Multiple lies in the administration about meetings with Russians or Russian-connected "businessmen". The pattern is clear: lie, then when you're caught, minimize, deflect or discredit.
* Repeated, public attempts to discredit the American intelligence community who is investigating him. Whatever the outcome of the investigation, the US needs the FBI/CIA/NSA etc. to have credibility in the world and at home.
* Recent confirmation that Trump is susceptible to blackmail over sex (Stormy Daniels), and also that he will attempt to cover up the blackmail.
* More and more of the Steele Dossier being confirmed by independent sources. The Dossier is largely about money laundering, contrary to what the MSM would have us all believe.
* Russians started tweeting about Trump's Presidential run in 2013, after a meeting he held in Russia. Seven days later, he registered the MAGA trademark. A couple of weeks later, the Miss Universe pageant, a Trump property, was awarded to Russia. Trump had an MOU for Trump Tower Moscow for more than a couple of years, which was only de-activated after he took office.
* A huge campaign by Russians to interfere with the 2016 election, and Trump's reluctance to admit such.
* Last but not least, four indictments of U.S. citizens who were on the campaign, with three deals made for pleas out of the four. And the recent indictments of Russian people and entities showing *some* of the election interference. This should be the beginning.
In general the MSM has done a truly incompetent job of reporting what's going on. I wouldn't even bother with CNN (hell no), NYT, or MSNBC except for a few good pieces here and there. If you watch Fox, you would barely know there's an issue with the President, but you might be convinced that Hillary Clinton is a threat. The Guardian does a good job, but they're a needle in a haystack. Similar the BBC. Seth Abramson just makes connections between stories that are well-sourced, and uses his experience as a criminal defense lawyer and a prosecutor to analyze what we know so far. He's biased, but he's rigorous and logical.