The attorney general of the United States can always be caught between the oath to uphold the Constitution and loyalty to the president who appointed him or her. But there is no choice — following the law and acting as a check on the executive are fundamental to our system of government.

Donald Trump has locked up a lot of firsts in disregarding expectations of presidential behavior, but his humiliation of Jeff Sessions, disparagement of other top Justice Department officials and warning to the special counsel probing Russian involvement in the 2016 election are frightening. As that investigation spreads to his family and his past business dealings, it’s clear Trump wants to stop the scrutiny.

There would be no quicker way to short-circuit his presidency.

In a lengthy, rambling and just plain weird interview with The New York Times on Wednesday, Trump faulted Sessions, one of his earliest and key supporters, for not telling him in advance that he would recuse himself from any investigation of Russia’s election meddling. Sessions removed himself in March after spending weeks resisting such demands.

Trump told the Times that the recusal was “unfair” to him. He also expressed displeasure with Sessions’ deputy, Rod Rosenstein, acting FBI Director Andrew McCabe and special counsel Robert Mueller, who is heading the probe.

Mueller was appointed by Rosenstein after Trump fired FBI Director James Comey because the president wanted to shut down the Russia investigation. With Sessions removed from the chain of command on the probe, Trump would have to order Rosenstein to fire Mueller, something the career prosecutor has reassured Congress that he would do only for “good cause.”

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So Trump appears to want to make that case. In the past few weeks, there have been rumblings from Trump’s inner circle of great dissatisfaction with Mueller, a highly regarded former FBI director. And that was way before his son, Donald Trump Jr., got tangled up in the probe.

Thursday morning, a stunned Sessions gamely said he would stay on “as long as that is appropriate,” and he said it with McCabe and Rosenstein by his side. “I am totally confident that I can run this office in an effective way,” he added. Start the countdown clock.

Later in the day, Trump’s spokeswoman said the president had no intention of firing Mueller, but that he wanted to make clear to the special counsel that he shouldn’t go beyond election interference. Earlier in the day, however, Bloomberg News reported that Mueller was doing exactly that. As he must.

Despite Trump’s denials that there was any collusion with Russia by his campaign or family, there is enough evidence to warrant a wide-ranging review. And that can’t be done without exploring the reasons why Russia might have wanted to help Trump. Would our rival now be advantaged in its relations because the president was beholden to Russian interests, either because of loans or because of shady real estate transactions? If that’s where the evidence leads, Mueller must follow.

Trump might never understand how his enormous power is restrained by the Constitution. But he should be smart enough to know that stopping Mueller would end that power altogether.