You understate the case.
Off the top of my head, we know the following suspicious pieces of information:
* Michael Flynn, national security advisor, forced to resign for failure to disclose conversations with Russian officials before taking office. He also failed to disclose money he had taken from Russia and/or Russians for a speaking engagement
* Carter Page, placed under surveillance because of a trip to Russia in July 2016. We, or at least I, don't know much, but apparently it was enough to raise some spook eyebrows because they obtained a warrant to intercept his communications and prompted them to take a serious look at links between the Kremlin and Trump.
* Paul Manafort was basically just a flat out Kremlin lobbyist
* Sessions failed to disclose two meetings with a Russian ambassador.
* Trump Jr met with someone claiming to represent Russia offering dirt on Clinton
* Then we come to the Steele dossier. To answer your earlier question about it, a couple of minor facts in it - conversations which it says took place - have been verified by the US intelligence community. Nothing else has. If you want my opinion, it contains a kernal of truth but the more sensational aspects (pee tapes) of it will turn out not to be true. It was compiled by a credible person, so I think it was put together by the intelligence community to put Trump on notice.
* Speaking of the intelligence community, they're leery of Trump and some reports have been that they've basically been refraining from sharing intelligence with the Whitehouse because they do not believe it was secure. I believe that intelligence agencies do not do stuff like this without a jolly good reason, and they're in a much better position to know what's up than us
* Trump fired the head of the FBI in an attempt to stop the Russia investigation. Occam's razor says: he did it because he didn't like where it was going
* Trump asked Russia to hack into the DNC so he could have a look at Clinton's emails in the middle of a speech or something lol jk just kidding. cough.
* Trump shared classified intelligence with a bunch of Russians in a meeting in the Whitehouse
* Trump has numerous links with Russia going back decades, some of which are of the decidedly shady persuasion
On their own, none of these facts is particularly damning (at least to Trump himself, some might get some of his associates in very hot water) but together? Suss.
One suspicious looking connection with Russia is probably innocent, two is co-incidence... 10 or more is beginning to look a lot like treason.