Concrete proof has not emerged. Right now it's at the "numerous senior sources say..." level of reporting. The full answer is now known by Congresspeople but it's classified [0]. We'll see if it gets leaked. It's not proven but I certainly wouldn't say zero evidence; reports at this "truth level" happen all the time, and are widely circulated and trusted.

Circumstantially, I think strong evidence that this happened is the Trump team's behavior after he was briefed by the NSA director, Admiral Rogers. The day after their confidential meeting, all relevant activities were immediately moved out of Trump Tower.

Anyway, this is off-topic and is a testy topic as it is so I apologize for the tangent. On topic I agree it shouldn't be possible for anyone to do this, no matter their politics.

[0] http://www.washingtonexaminer.com/byron-york-yes-congress-ha...

The second sentence in the linked article is this: "The Foreign Intelligence Surveillance Court issued the warrant, the official said, after investigators determined that Mr. Page was no longer part of the Trump campaign, which began distancing itself from him in early August."