Jeff Sessions’s agenda could erase many of the legal gains of the civil-rights movement.

“While I have opposed many of the actions taken by Attorney General Sessions, it would be unacceptable for the president to fire him now in order to install someone willing to subvert the Mueller investigation,” Senator Dianne Feinstein of California, the top Democrat on the Judiciary Committee, said in a statement Thursday. “I, for one, believe Congress must do all it can to stop such an act.” Senator Chris Murphy of Connecticut, another Sessions critic, urged Democrats to go to the polls in November in part to protect a man he voted against last year. “If Democrats don’t win, Trump is emboldened, Sessions and Mueller are gone,” Murphy tweeted Friday. “We slide closer to a banana republic.”

Under pressure from Democrats after his previously undisclosed meetings with the Russian ambassador became public last year, Sessions announced in March 2017—just six weeks after taking over the Justice Department—that he was recusing himself from the federal investigation into Russian interference in the 2016 election and whether the Trump campaign was involved. That move paved the way for Deputy Attorney General Rod Rosenstein to appoint Mueller as special counsel after Trump fired FBI Director James Comey later that spring.

Because Rosenstein is now overseeing the Mueller probe, replacing Sessions would mean the new attorney general would have the authority to restrict or end the investigation.

Sessions’s recusal infuriated Trump, and their relationship has never been the same. Several times over the past year, the president has lashed out at the attorney general in interviews or over Twitter, accusing him of disloyalty and saying he never would have appointed him if he knew he was planning to recuse himself. The latest volley occurred this week, in the aftermath of the conviction of Paul Manafort, Trump’s former campaign manager, and the guilty plea by Michael Cohen, his longtime personal attorney.

“He never took control of the Justice Department,” Trump said in a Fox News interview broadcast Thursday. He again faulted Sessions for stepping aside and challenged him to investigate Democrats and other Trump critics for “collusion,” the same allegations dogging the president and his associates.

Sessions has mostly laid low in the face of the president’s taunts, but on Thursday he issued a rare statement defending himself. “I took control of the Department of Justice the day I was sworn in,” he said, listing areas, like immigration enforcement, where he has clearly advanced Trump’s agenda. “While I am attorney general, the actions of the Department of Justice will not be improperly influenced by political considerations. I demand the highest standards, and where they are not met, I take action.”