We don’t often have a “war of words” between the President and the Chief Justice, but that is what happened just before Thanksgiving. A California federal judge ruled that under federal law, migrants can seek asylum anywhere on U.S. soil, even if they did not enter through an authorized checkpoint. Trump called him an “Obama Judge,” adding “it’s not going to happen like this anymore.”

Chief Justice Roberts reacted quickly and clearly. “We do not have Obama judges or Trump judges, Bush judges or Clinton judges.” To which Trump tweeted, “Sorry Chief Justice Roberts, but you do indeed have ‘Obama judges.’”

Maybe this is a new form of “checks and balances.”

As it happens, a day or two later a Trump-appointed judge ruled against Trump and reinstated the White House press credentials of CNN’s Jim Acosta. If the President took note of that judge’s provenance, he didn’t say so.

But this column isn’t about the President, it’s about John G. Roberts, Jr., who in 2005, at age 50, became our 17th Chief Justice. President George W. Bush appointed him to that position.

Since then, he has presided over an increasingly divided and politicized Supreme Court, where Republican and Democratic appointees mostly vote as separate blocs. Roberts has been identified with the “conservative” wing of the Court, usually siding with Justices Thomas, Alito, Scalia, and Kennedy, the latter two now succeeded by Gorsuch and Kavanaugh. He has often led that group, assigning to himself such opinions as Trump v. Hawaii, where the Court upheld the Trump travel ban.

No one claims, however, that Roberts is a conservative “hack.” He is regarded, by general consensus, as an exceptionally smart person who cherishes the institution that he leads. And it increasingly appears that if not a “secret liberal,” he is at least an open-minded jurist who takes each case as it comes and has become a moderating force, perhaps even the “swing” vote, on the Court.

The first sign of this appeared in 2012, when Roberts cast the deciding vote to uphold the Affordable Care Act (Obamacare), albeit on grounds different from the other four members of the majority. That vote told us that he does not march in lockstep with his fellow conservatives, and his judicial positions since then seem to reflect a careful judge determined to call constitutional balls and strikes (as he put it during his confirmation hearings) impartially and to uphold the rule of law.

If Roberts has, in fact, shifted somewhat ideologically, he would be following in the footsteps of several previous justices – Chief Justice Warren (“biggest damn fool mistake I ever made,” said President Eisenhower), as well as Justices Blackmun, Stevens, Kennedy, and Souter. All were appointed by Republican presidents, all are now regarded as somewhere between “moderate” (Kennedy) and “liberal” (Stevens).

This is a non-partisan two-way street. Roosevelt appointed Felix Frankfurter, Kennedy appointed Byron White. Neither turned out to be liberal Justices.

Not to be misunderstood, Chief Justice Roberts is undoubtedly a “conservative.” He was in the majority in Citizens United, and he wrote the opinion in Shelby County v. Holder, which invalidated much of the Voting Rights Act. He dissented in Obergefell v. Hodges, the gay marriage case. Last year, in Gill v. Whitford, he expressed skepticism that social science could provide a means of remedying partisan gerrymandering. Trump v. Hawaii is but one of many cases in which he has been highly deferential to the power of the Executive.

Still, this member of the Court seems to understand that not everything is black or white, and in last year’s Supreme Court term, Roberts can be found on both sides of the ideological divide. He was with the other court conservatives in the Janus case, which decided that requiring a non-union public employee to pay a fee in lieu of union dues violates the First Amendment; but he was with the liberals in the Carpenter case, which held that the government must obtain a warrant in order to obtain cell phone site location information. His last words in the recent exchange with the President bear repeating. “The independent judiciary is something we should all be thankful for.”

We usually look at what the Court decides, not what it declines to consider. But last Monday, the Court sided with Planned Parenthood and turned down requests to review lower court decisions dealing with when states can terminate Medicaid contracts. It takes four Justices to hear a case, and three – Thomas, Alito, and Gorsuch – wanted to do so. Roberts did not, and neither did the newest member of the Court, Brett Kavanaugh. Of course we don’t know how they would vote on the merits of such a case, but in this instance the decision not to decide shows, once again, that the Chief Justice goes his own way. It’s too soon to tell about Justice Kavanaugh, but he could have cast that crucial fourth vote and did not do so.

How Chief Justice Roberts leads the Supreme Court is of momentous importance. Ours is a “government of laws and not of men.” John Adams wrote those words into the Massachusetts Constitution in 1780, and they remain true today. That is neither a liberal nor conservative principle, it is an axiom of democracy. And when the Chief Justice said, “We do not have Obama judges,” that is what he was trying to explain to the President. Is he right? Or is Trump right? As we veer ever closer to what the pundits call a “constitutional crisis,” we may find out.