Don McGahn, then White House counsel, attends a Senate hearing in 2018. (Brendan Smialowski/AFP/Getty Images)

Given his own characteristic style, it's not really clear how earnestly President Trump was suggesting that he'd try to block former White House counsel Don McGahn from testifying before Congress.

During an interview with Fox News’s Catherine Herridge he sort of hinted that he’d exert executive privilege to block McGahn’s testimony but ... it’s not entirely clear.

"Well, I’ve had him testifying already for 30 hours,” Trump said of McGahn, referring to McGahn's extensive interviews by special counsel Robert S. Mueller III's team. “I don't think I can let him and then tell everybody else you can,” Trump continued, “because especially him because he was a counsel."

"So, as far as you’re concerned, it’s really — it’s kind of done, it’s done?” Herridge asked.

"I can’t say, well, one can and the others can't,” Trump replied. “I would say it's done,” he added.

We can certainly say that this exchange suggests Trump is not enthusiastic about McGahn testifying, for understandable reasons. McGahn’s interactions with Trump related to an effort to fire Mueller were a central part of Mueller’s consideration of obstruction of justice by the president.

We also know that his attorneys, specifically Emmet Flood, believe that Trump has the right to try to block congressional testimony. In a letter Flood sent to Attorney General William P. Barr after the release of the Mueller report last month, he argued that it was “one thing for a President to encourage complete cooperation and transparency in a criminal investigation conducted largely within the Executive Branch; it is something else entirely to allow his advisors to appear before Congress, a coordinate branch of government, and answer questions relating to their communications with the President and with each other.”

So let's say that this is Trump's plan: Block McGahn from testifying on the grounds that his testimony would be covered by executive privilege. Is that possible?

We need to first articulate what executive privilege is and isn't. It isn't a blanket, inviolate protection covering any interaction that takes place between the president and his staff. It is, instead, a concept established by the Supreme Court during the Nixon administration recognizing that certain internal executive branch communications should be allowed to be kept private. In essence, a claim of executive privilege amounts to an appeal to a court to allow certain communications to remain out of the public eye. Whether a court agrees that the communications should be privileged comes down to the court, the content of the communications and the reason they're being sought.

The problem for Trump, as Aaron Blake pointed out last month, is that you can't justifiably exert privilege over communications that are already public. Even if Trump didn't explicitly waive privilege, he can do so implicitly.

“The waiver, however unwise, cannot be pulled back,” former federal prosecutor Gene Rossi told Blake. “ ‘I consented to a search of my car and withdraw it after the police find a kilo of heroin?’ Illogical.”

Last week, I spoke with Duke University professor of law Lisa Kern Griffin about the broad effort by the White House to stymie Democratic investigations. During that conversation, the possibility of Trump trying to withhold testimony by McGahn came up, including his testimony about Trump’s desire to have Mueller fired as special counsel.

“The issues that Congress wants to talk to Don McGahn about involve either no privilege or areas where privilege has already been waived,” she said at the time. “So I don’t anticipate that any assertions of privilege will be meaningful with respect to Don McGahn’s testimony.”

After the Fox News interview, I reached back out to her over email where she reiterated that point.

"Once he was permitted to reveal the content of conversations about terminating Mueller in one context, there is no more claim of privilege in those conversations."

She also returned to another point she'd made originally. Trump has a particular disadvantage with McGahn: The guy no longer works for the administration.

"The White House no longer exercises supervisory control,” she pointed out. “Current members of the administration have to fear employment consequences from testifying. But there are no enforceable 'non disclosure agreements' in this context."

So what's the plan? Tie it up in the courts.

“I believe any court considering the issue would enforce the subpoena,” she said. “But forcing the issue into court delays the testimony. These standoffs will continue for as long as the White House can sustain them.”

It's hard to say how long that would take.

“Clashes between branches of the government implicate core constitutional questions, and the arguments back and forth, as well as the courts’ decisions, will take some time,” she said. “We can expect the president to use the courts as a shield and delay at every turn, a tactic he has employed in his business dealings as well.”

As she noted last week, subpoenas and any contempt citations that might arise during the fight between the White House and Congress are set to expire in early 2021, unless the next Congress renews them. At the very least, Trump likely hopes to kick the fight out past 2020 — or, Griffin figures, to perhaps goad Democrats into starting an impeachment fight.

So the answer to the main question — can Trump keep McGahn from testifying (if that’s what he was hinting at with Herridge)? — is likely no. But he can possibly keep him from testifying long enough to either (1) outlast the current Congress and its request for testimony or (2) stretch past the reelection campaign.

Either of which, it’s safe to assume, would be good enough for Trump.