And here it is, the first big Trump Cabinet shakeup of the post-midterms season: Jeff Sessions just resigned from his job as attorney general. And it appears the decision wasn’t voluntary. In his letter of resignation, Sessions wrote that President Trump had asked him to step down. But Sessions’s departure comes as no surprise to political observers. Trump has long been angry with his attorney general, once one of his closest supporters. Arguably, the bad blood started between the two once Sessions recused himself from any investigation involving the 2016 campaign, which turned out to include special counsel Robert Mueller’s investigation into possible coordination between the 2016 Trump campaign and Russia. But now that Sessions is actually leaving, the fate of Mueller’s probe looks a lot more uncertain.

One of the most immediate consequences of Sessions’s resignation is that his replacement, Acting Attorney General Matthew Whitaker, now replaces Deputy Attorney General Rod Rosenstein as the supervisor of Mueller’s probe. This is potentially a big deal for Mueller’s investigation because had Sessions been fired, it wouldn’t have been clear whether Trump could replace him with someone like Whitaker, who until today was Sessions’s chief of staff, which is not a Senate-confirmed position or high enough in the Justice Department hierarchy.

As supervisor, Rosenstein has overseen Mueller since the beginning of the investigation, and he has defended Mueller’s work at crucial moments, such as when Republicans in Congress questioned whether the probe is biased. After rumors spread in September that he was about to be fired, he reiterated that he thought the investigation was “appropriate and independent.” He has also pushed back on congressional requests for documents related to the investigation, even under significant duress.

Whitaker, on the other hand, appears to have very different views about the proper scope of Mueller’s investigation. In an op-ed for CNN last year, Whitaker wrote that Mueller was overstepping his bounds by looking into financial records related to the president’s businesses and argued that Rosenstein should rein in the special counsel. And in a TV appearance last year, Whitaker said the attorney general could decrease the special counsel’s budget “so low that his investigation grinds almost to a halt.”

This matters because the person overseeing the Mueller investigation has significant power over both its scope and direction. Rosenstein chose to protect Mueller, but Whitaker could do the opposite. If Whitaker — as he appears — is more skeptical of Mueller’s investigation than Rosenstein was, there are avenues for him to slow or curtail the investigation as it enters what may be its final, crucial stages. When Mueller files his report, the person overseeing him has significant authority over how much information from that report is disclosed to Congress or the public. And, of course, the power to fire Mueller is now in Whitaker’s hands, too.

On the other hand, as I wrote earlier this year, we shouldn’t jump to conclusions either. Trump needs to tread carefully when it comes to Mueller. A blatant attempt to intimidate the special counsel or interfere with Mueller’s work could constitute obstruction of justice by Trump, which Mueller already appears to be investigating.

But the bottom line is this: Mueller’s investigation is on shakier ground than it was only a few hours ago, even though it’s not yet clear if — or how — Whitaker might try to constrain his work. That said, the stakes are now considerably higher for whatever new findings come out of Mueller’s investigation, which is poised to move back into the spotlight now that the midterms are over.