John Yoo has an unusual vantage point on two key questions that will burn through the Senate’s confirmation hearings on Supreme Court nominee Brett Kavanaugh, which begin Tuesday.

The first: How much power does the Constitution give the president? And, more important: How much power does Kavanaugh believe the Constitution gives the president?

In particular, this president.

While the loudest and most passionate arguments at the confirmation hearings will be over abortion rights, the debate over presidential power could have more immediate ramifications. That debate could someday determine the fate of President Trump, as the investigation into whether his 2016 campaign colluded with Russia grinds on toward a possible showdown in the courts.

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Yoo’s vantage point is unusual because of a pair of biographical details: He’s a longtime friend of Kavanaugh and a lifelong Republican who loathes Trump.

Yoo thought so little of Trump that he considered voting for a Democrat for the first time in his life. Yoo called the Republican nominee a “demagogue” before the 2016 election, and he still worries about the damage Trump’s “lack of impulse control” could wreak, especially when it comes to national security.

“I won’t say who I voted for, but I didn’t vote for Trump,” Yoo said on my “It’s All Political” podcast, recorded in his office at the UC Berkeley School of Law, where he is a constitutional law professor and expert on presidential power.

Yoo, 51, is best known for his willingness to stretch Oval Office power to its most lethal limits. When he worked in the Department of Justice in the months after the Sept. 11 terrorist attacks, Yoo wrote a legal memo that President George W. Bush’s administration used to authorize what it called “enhanced interrogation” techniques such as waterboarding on terror suspects in U.S. custody.

Many — including the late GOP Sen. John McCain — called those techniques torture. Some still think Yoo should be prosecuted as a war criminal for crafting the memo. Protesters still heckle Yoo wherever he speaks. He remains unapologetic, believing that the president has broad powers to act in the name of national security.

“I love it. I love that they hate me,” Yoo said of his critics.

“The most important thing is that if you’re getting criticism and feedback and protests, you should be more forthright and explain it,” Yoo said. “And the worst thing would be to allow people to shut down different points of view.”

That brings us to Kavanaugh, Bush’s former staff secretary whom he later appointed to be a judge on the U.S. Court of Appeals in Washington, D.C. Some Democrats worry that Kavanaugh, like his Yale Law School classmate Yoo, believes that a president packs an extraordinary amount of executive power.

But Kavanaugh’s critics are not worried about Trump using that power to reinstate waterboarding — even though the president has said he approves of “a lot more than waterboarding.” Democrats are more immediately concerned that Kavanaugh won’t think Trump has to comply with a subpoena from Special Counsel Robert Mueller to talk about the 2016 campaign and many, many associated issues.

They also worry that Kavanaugh won’t think Trump, as a sitting president, should have anything to answer for in the Michael Cohen case. Trump’s ex-lawyer said the president directed him to violate federal campaign laws by ordering him before the 2016 election to pay hush money to women he allegedly had sexual affairs with.

Yoo said people shouldn’t worry about Kavanaugh going easy on Trump.

“I would bet money — a lot of money, actually — that Kavanaugh would say that the president doesn’t have any immunity” from things he did before he was president, Yoo said.

And what if Trump refuses to answer a subpoena from Mueller? Does Kavanaugh agree with the precedent set by the court’s Watergate-era ruling that Richard Nixon had to comply with a subpoena to turn over the Oval Office recordings that, in the end, doomed his presidency?

“I would expect Kavanaugh to uphold that, too,” Yoo said.

Democrats are dubious. They point to a 2009 Minnesota Law Review article in which Kavanaugh wrote that presidents shouldn’t have to respond to civil lawsuits and criminal investigations until they leave office.

“Like civil suits, criminal investigations take the president’s focus away from his or her responsibilities to the people,” Kavanaugh wrote. “And a president who is concerned about an ongoing criminal investigation is almost inevitably going to do a worse job as president.”

That sounds a lot different from what Kavanaugh was saying a decade earlier. Back then, he was an attorney on special prosecutor Kenneth Starr’s team investigating then-President Bill Clinton for various scandals.

Kavanaugh aggressively argued that the president should have to testify before a special prosecutor. He even drew up a list of 10 explicit questions to ask Clinton about his alleged sexual interaction with White House intern Monica Lewinsky, including, “If Monica Lewinsky says that you had phone sex with her on approximately 15 occasions, would she be lying?”

Yoo reads Kavanaugh’s mixed signals on executive power differently. He pointed out that in the law review article, Kavanaugh also agreed with the Supreme Court’s 1997 Jones vs. Clinton decision. In that case, the court ruled that the Constitution didn’t protect a president from responding to lawsuits, a ruling that Kavanaugh wrote “may well have been entirely correct.”

That dispute aside, Yoo suggested that senators ask Kavanaugh a “deeper constitutional question” during the confirmation hearings about presidential power. A question that truly would test its limits — and Trump’s willingness to shatter them.

What would happen, Yoo said, if the Supreme Court were to rule that Trump must comply with a subpoena to testify, “and Trump says, ‘Go screw yourselves. I’m not going to do it’?”

Then what?

Even Nixon ultimately complied with the court’s demand that he turn over the Watergate tapes. But Nixon, who had served in the House and Senate and as vice president, believed in the institutions of government. At least at some level.

“I’m worried that Trump does not have that background,” Yoo said. “Nixon is starting to look better and better by contrast these days.”

That’s small comfort to Democrats watching this week’s hearings, knowing they don’t have the votes to derail Kavanaugh’s nomination. It is another reminder that the 2016 election had far-reaching consequences, for Trumpers and Never Trumpers alike.

Joe Garofoli is The San Francisco Chronicle’s senior political writer. Email: jgarofoli@sfchronicle.com Twitter: @joegarofoli