It should be clear to all Americans why President Donald Trump fired Attorney General Jeff Sessions. The former Alabama senator, the first major elected official to stick his neck out for candidate Trump, nevertheless chose duty over fealty when he recused himself from involvement in the Russia investigation.

Sessions suffered mightily for that choice, taking verbal beatings and public humiliation from the president for more than a year before he was asked to resign on Wednesday.

His dismissal should have resulted in Deputy Attorney General Rod Rosenstein, the second-highest-ranking official in the department, assuming the role of acting AG. But Rosenstein, of course, supervised the Russia investigation. So Trump bypassed him in favor of Matthew Whitaker, Sessions’ chief of staff, who has never been confirmed by the Senate and who has been openly contemptuous of the investigation.

Whitaker has already made abundantly clear, in a CNN opinion piece published in August 2017, that Rosenstein should “order [Robert] Mueller to limit the scope of his investigation,” and in an earlier interview said he could “see a scenario where Jeff Sessions is replaced with a recess appointment and that attorney general doesn’t fire Bob Mueller, but he just reduces his budget so low that his investigation grinds to almost a halt.” As of Wednesday, Whitaker became Rosenstein’s boss and overseer of the investigation.

This is precisely the moment at which Congress must exercise its check-and-balance authority to preserve the integrity of Special Counsel Robert Mueller’s investigation, and that includes its funding, without which the investigation will become a shell.

So far, the reaction from the Republican-controlled Senate has been mostly silence or capitulation, including from Trump toady Lindsey Graham, R-S.C., who in an earlier incarnation vowed there would be “holy hell to pay if Jeff Sessions is fired.”

The appropriate reaction came from the now-flipped House, where incoming Democratic chairs of the Judiciary, Intelligence, and Oversight committees have already sent a stern letter to Whitaker requesting that all materials related to the Mueller investigation be preserved and outlining steps needed to protect the integrity of the probe.

To fulfill its obligation, not to the president, but to the American people, both the House and Senate should act quickly to do just that. Trump has threatened and intimidated for far too long on an investigation that is crucial to determining the validity of the 2016 election.