Watch: Legal expert Laurence Tribe explains how Trump’s ‘collusion’ with Russia has already been proven
Prof. Laurence Tribe of Harvard Law School on MSNBC.

Harvard Law School constitutional law scholar Laurence Tribe offered a fascinating theory as to why President Donald Trump's collusion with Russia has been proven.

"How damning does the evidence from Robert Mueller need to be, and do you think he is on something of a stopwatch right now, that there is a limited amount of time for him to come out with this report and still maintain public confidence? MSNBC anchor Katy Tur asked. "Do you think that's eroding with all the president's attacks?"

"The president's attacks are certainly taking a toll," Tribe replied. "But I think fundamentally putting a stopwatch on this kind of investigation is against the interests of all of us, of the United States."

Professor Tribe cited a recent op-ed column by former CIA director John Brennan.

"Certainly we know there has been collusion, as Brennan said in that remarkable op-ed," he continued.

"Hold on," Tur interrupted. "Hold on."

"You said we know there has been collusion. I think there are those out there that would argue that we do not know that," Tur challenged.

"What I think they mean is that is we don't know exactly what responsibility the president had for actually conspiring in a criminal way with Russians," he answered. "But as Jen [Rubin] says, you don't need criminal conspiracy -- what you need is abuse of power."

"And it is an abuse of power of the first order to cooperate with a foreign adversary, to seize the presidency of the United States, and then to make it hard for the American people to discover exactly who did what and how best to stop it," Tribe explained.

"That is the fundamental issue and it's an issue that goes to the free speech rights of those the president attacks and the national security of this country," he added. "Nothing could be more serious."