Jonathan Chait lays out the evidence that we know about, and concludes that President Trump is almost certainly colluding with Vladimir Putin:
A case like this presents an easy temptation for conspiracy theorists, but we can responsibly speculate as to what lies at the end of this scandal without falling prey to their fallacies. Conspiracy theories tend to attract people far from the corridors of power, and they often hypothesize vast connections within or between governments and especially intelligence agencies. One of the oddities of the Russia scandal is that many of the most exotic and sinister theories have come from people within government and especially within the intelligence field.
Suppose we are currently making the same mistake we made at the outset of this drama — suppose the dark crevices of the Russia scandal run not just a little deeper but a lot deeper. If that’s true, we are in the midst of a scandal unprecedented in American history, a subversion of the integrity of the presidency. It would mean the Cold War that Americans had long considered won has dissolved into the bizarre spectacle of Reagan’s party’s abetting the hijacking of American government by a former KGB agent. It would mean that when Special Counsel Robert Mueller closes in on the president and his inner circle, possibly beginning this summer, Trump may not merely rail on Twitter but provoke a constitutional crisis.
[I]f you’re Putin, embarking upon a coveted summit with the most Russophilic president since World War II, who is taking a crowbar to the alliance of your enemies, why wouldn’t you help him in 2018 and 2020? Ever since the fall of 2016, when Republican Senate Majority Leader Mitch McConnell privately turned down an Obama-administration proposal for a bipartisan warning to Russia not to interfere in the election, the underlying dynamic has been set: Most Republicans would rather win an election with Putin’s help than lose one without it. The Democrats, brimming with rage, threaten to investigate Russian activity if they win a chamber of Congress this November. For Putin to redouble his attack — by hacking into voting machines or some other method — would be both strategic and in keeping with his personality. Why stop now?
It's straightforward and logical, as Occam's Razor should be. And it's deeply frightening.