Ordinarily, it would be big news if a president denounced one of his own appointees in public. But with President Trump and Attorney General Jeff Sessions, this has been the normal order of things for months.

The disagreement between the two men began when Sessions, immediately upon taking office, announced that he was recusing himself from the nascent investigation into Russian interference in the 2016 election. His reason was that he had himself been part of Trump’s presidential campaign — in fact, the first major officeholder to jump on the Trump train. He had also had a couple of very casual encounters with Russians that were insignificant but could have become an excuse to question the legitimacy of the investigation, of Sessions’ office, or even of the very Justice Department.

Sessions reasonably concluded that, in order to prevent any appearance of impropriety, he shouldn’t have a personal role in deciding where the Russia investigation went. The recusal seemed like a good start to his term as attorney general.

And Trump has never forgiven him for it.

It is in this context that Trump tweeted angrily on Tuesday, referring to special counsel Robert Mueller’s investigation, “If we had a real Attorney General, this Witch Hunt would never have been started!”

Sessions would be well within his rights to quit after this. Perhaps he ought to, for the sake of whatever personal dignity Trump has left him with after a year of public abuse. And perhaps that’s exactly what Trump wants.

But Trump should really think twice about what happens if Sessions leaves. Because it won't be good for him.

Trump’s best play has always been to let this investigation take its course, and the faster the better. It is increasingly evident that Russian interference — which was very real — not only had nothing to do with Trump himself, but also had nothing to do with the election outcome.

Unfortunately, Trump is too upset the investigation even exists to take this wise perspective. He believes the investigation is an insult that somehow denigrates his achievement in winning the presidency against all odds.

If Sessions leaves, he will not be easy to replace right now. In fact, Sessions’ resignation would be an unmitigated disaster for Trump. Trump may never succeed in confirming another attorney general, in part because of public statements like the above tweet.

Imagine, for a moment, that Sessions chose to resign today, feeling unable to do his job amid Trump’s disparaging tweets. What happens next?

Trump just strongly implied on Twitter that a “real Attorney General” would end the Russia investigation for him. In that context, every single senator from both parties will want to know whether his nominee for the position can be trusted not to interfere with the investigation. And with so many Republican Trump allies in the Senate on record opposing Sessions’ firing , Mueller’s firing , or any other move by Trump to quash the investigation, Trump has a zero percent chance of successfully appointing someone he can rely on to do anything like that. Also, Congress can prevent him from making a recess appointment by simply holding pro forma sessions year-round, as the Supreme Court ruled during Barack Obama's presidency.

Meanwhile, given Sessions' unpleasant experience, it's very, very unlikely that any self-respecting person intent on refusing to fire or interfere with Mueller will want the job. And even if such a person did exist and was interested in the job, his appointment would leave Trump right back where he started — saddled with an attorney general who won't do anything about the "witch hunt."

If, as is likely, Trump refuses to appoint someone who will pledge to leave Mueller alone, he will end up with an acting attorney general. Here, too, he has little latitude. He can appoint other Senate-confirmed officials to serve for as long as seven months, but a little-noticed 2017 Supreme Court decision ensures that the only person who can be appointed as permanent acting attorney general is Deputy Attorney General Rod Rosenstein. Trump and Rosenstein have an ... interesting relationship already.

In short, President Trump always had fewer options in this matter than he thought. And now that he’s signaled to the Senate that he’d like an attorney general who will stop the Mueller investigation, he has fewer options still. Unless he wants to create a major unnecessary and distracting controversy that will sidetrack his presidency, he should be rooting for Sessions to stick around.