× Expand Alex Brandon/AP Images Attorney General William Barr speaks about the census as Commerce Secretary Wilbur Ross listens during an event with President Donald Trump in the Rose Garden.

You might have missed it, but the most important moment at Trump's Thursday joint press conference with Attorney General William Barr was the one where Barr committed Trump to obeying the Supreme Court.

By making an explicit commitment to obey the courts and the rule of law, Barr has boxed in his president. Trump, at various points, has suggested that he might defy or circumvent the courts. But Barr this time left no wiggle room.

In discussing how the administration was dropping its efforts to add a citizenship question, because there was no time to pursue appeals and still comply with the Constitutional census deadline and high court ruling, Barr said this (read it slowly and carefully):

Some in the media have been suggesting in the hysterical mode of the day that the administration has been planning to add the citizenship question to the Census by exec fiat without regard to contrary court orders or what the Supreme Court might say. This has been based on rank speculation. As should be obvious, this has never been under consideration. We have always accepted that any new decision to add a citizenship question to the census would be subject to judicial review.

That was, to put it mildly, BS. In fact, Trump has repeatedly threatened to defy the courts, most recently when he abruptly changed the game plan on the Justice Department’s efforts to follow the Supreme Court’s census ruling.

What is Barr up to? He has been a total sleaze in his efforts to marginalize Robert Mueller and support other extravagant Trump claims of unlimited executive power.

But even Barr, evidently, has his limits. The commitment to obey the courts, sandwiched in between layers of craven flattery, was a message directed squarely at Donald Trump.

Over the past six months, Trump has managed to get rid of minders in favor of enablers. He has also built a record of unceremoniously dumping one official after another when they no longer served his interests. (See Acosta, Alex).

But Barr is one of the few officials who would be very hard to dump, because he has been so deft at defending outrageous policies while seeming reasonable. But Barr draws the line at directly flouting the Supreme Court.

So yesterday's victory was doubly sweet. Not only did Trump cave on the census citizenship question, but his attorney general declared unambiguously that the White House would not defy the courts. In the Trump era, that passes for a win for the rule of law.

Let's be clear. I don't trust Barr (or Trump) as far as I can throw them, nor should you. But yesterday's declaration makes it just a little harder for Trump to pretend that he is above the law—and adds one more potent impeachment count should he go back on Barr's pledge.