Senator Chuck Schumer says he knows why President Trump ended up picking Judge Brett Kavanaugh for the Supreme Court. Mr. Schumer, according to the dispatch in the New York Post, is “worried” about Special Prosecutor Robert Mueller’s probe and reckons Judge Kavanaugh would be a “barrier” to the investigation. All we can say is that if it’s true, it’s the best of the many wonderful things people are saying about Judge Kavanaugh.

The judge has certainly been in the thick of the fight to defend separated powers, which are so threatened by the very concept of independent (and special) counsels. Since the presidency of Richard Nixon, these extra-constitutional prosecutors have been used to harry the president, more often, it has seemed, Republicans. Abuses by such prosecutors and independent regulators, have, in our view, precipitated a crisis over separated powers as a foundation of our freedom.

Judge Kavanaugh gets this down to the ground. That may have something to do with the fact that he worked for the office of the independent counsel. That was back in the 1990s, when the independent counsel was Kenneth Starr. We view Judge Starr as a fine, high-integrity individual. Nonetheless we opposed his pursuit of President Clinton. Our view was — and is — that no prosecutor can target a president. That is impeachment, a power that belongs solely to the House.

The Supreme Court has so far been weak on this. In the 1980s, in a case called Morrison v. Olson, the Nine okayed the independent counsel act that was later used by Judge Starr to pursue Mr. Clinton. Justice Antonin Scalia, though, wrote a historic, if lone, dissent, in which he warned that unleashing a special prosecutor — with a vast team and unlimited budget to pursue a single target — risked affecting the “boldness of the president.”

That, by the way, came to pass in the late 1990s, when President Clinton was being harried by the independent prosecutor or recovering from the impeachment the prosecutor precipitated. The 9/11 commission itself concluded that aides shrank from alerting Mr. Clinton to the chance to launch a preemptive strike against Osama bin Laden because the president was so distracted by troubles arising from the scandal Judge Starr used to try to oust him from office.

So why is Senator Schumer so worried about Judge Kavanaugh? It turns out that once Judge Kavanaugh acceded to the District of Columbia appeals circuit, he emerged as a defender of separated powers and a protector of the presidency as an constitutional concept. He was particularly strong in a case challenging the power of Congress to limit the reasons for which the president could dismiss the director of the Consumer Financial Protection Bureau.

In that case, Judge Kavanaugh pointedly cited Justice Scalia’s dissent in Morrison. He did so with an audacity that was breathtaking (and at the time well marked by Stuart Benjamin in the Washington Post’s law blog). It must be one of the few times in history that a dissent was cited as precedent. Judge Kavanaugh’s opinion in that case was reversed by the full circuit, a reversal that will no doubt come up in his confirmation hearings.

Judge Kavanaugh was particularly effective, in the view of Mr. Benjamin, in attaching importance to the fact that when the independent counsel law came up for renewal there was, in the words of Judge Kavanaugh, “nearly universal consensus that the experiment had been a mistake and that Justice Scalia had been right back in 1988 to view the independent counsel system as an unconstitutional departure from historical practice and a serious threat to individual liberty.”

It could be called Kavanaugh’s finest hour. We are not privy to President Trump’s motives. It, though, is clear as a bell to us that Justice Scalia was, and Judge Kavanaugh is, ahead of the constitutional curve. The thing for both sides to remember is that separated powers is a doctrine that protects both Democrats and Republicans, and Judge Kavanaugh’s view will protect all presidents, from whatever party. No wonder Senator Schumer is so worried.