Special counsel Robert Mueller’s investigation of allegations of collusion between President Donald Trump’s 2016 campaign and Russian political and business interests increasingly looks like it is coming up empty, a pair of legal experts said Monday.

Harvard University law professor Alan Dershowitz (pictured above left) said on “The Laura Ingraham Show” that Trump should welcome a shift from collusion to obstruction of justice, as Axios recently reported is taking place.

“That’s the best thing that could possibly happen to President Trump, because there, we don’t have a criminal charge,” he said. “You basically have a constitutional crisis.”

The crux of the potential allegation against Trump is that his firing of FBI Director James Comey last year — a power conferred upon him as the chief executive by the Constitution — amounts to an abuse of power if he had a corrupt reason for doing so.

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The last time Congress impeached a president on those grounds was in 1868, when radical Republicans objected to President Andrew Johnson’s dismissal of his war secretary, Dershowitz said.

“Historians have all concluded that was a misuse of the impeachment power,” he said.

Noting that Comey used a Columbia University law professor to leak memos he prepared about his interactions with Trump as FBI director, Dershowitz said the president was right to fire him.

“The director of the FBI — one of his main jobs is to prevent leaks in other parts of the government,” he said. “That’s part of your job … If the end result of this whole thing is to finally put an end, put a lid on leaks, it will hurt journalists like you, because you live by leaks. But it will help governance enormously because there’s no accountability for off-the-record leaks.”

Joseph diGenova, a former U.S. attorney for the District of Columbia under President Ronald Reagan, said during a separate appearance on the Ingraham program that Mueller’s probe should end.

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“Whatever purpose it had in the beginning, that purpose has now been served, and it is time for this matter to end,” said diGenova (pictured above right).

In fact, diGenova said, there never were legitimate grounds to appoint a special counsel in the first place. He blamed Deputy Attorney General Rod Rosenstein, who gained control over the Russia investigation after Attorney General Jeff Sessions recused himself due to his involvement in the Trump campaign.

“Rod Rosenstein knew exactly what he was doing when he appointed Mueller without any limitations on his jurisdiction and without any evidence that a predicate crime had been committed,” diGenova said. “There was no crime. There was no reason to appoint a special counsel. Rosenstein knew that. I believe he acted purposely and willfully in order to set this in motion.”

DiGenova also blasted former CIA Director John Brennan, who criticized Trump over the weekend following Friday’s firing of former FBI Deputy Director Andrew McCabe. Brennan condemned Trump in a tweet for “venality, moral turpitude, and political corruption” and declared that America would “triumph” over the president.

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“The tweet is revelatory for a number of reasons. I have never doubted the partisanship of Brennan, but I thought he was a little smarter than this. Because what this does now is show he had an animus toward Trump while he was the CIA director,” he said.

“And it actually makes it more obvious that there has to be a grand jury to investigate the unmaskings and the leaking of the name. When you hate the way John Brennan hated in that tweet, you are capable of very, very bad things,” diGenova continued.

But Brennan’s tweet could be a “blessing in disguise,” he added. “It really showed how, how horrible these people are, just absolutely horrible.”

PoliZette senior writer Brendan Kirby can be reached at [email protected]. Follow him on Twitter.