But as acting attorney general, Mr. Whitaker would be in a position to impede or undermine the investigation or to block Mr. Mueller from delivering a final report on whether Mr. Trump’s campaign advisers conspired with Russia to influence the 2016 campaign, and whether the president tried to cover it up.
Any such step could set off a dramatic clash with the new Democratic majority in the House. Representative Jerrold Nadler, Democrat of New York, who will become the chairman of the House Judiciary Committee, was one of several Democrats to promise investigations once the party takes control in January.
“The American people understand that no person is above the law and have demanded accountability from their government,” Mr. Nadler said. “The firing of Jeff Sessions will be investigated and people will be held accountable. This must begin immediately, and if not, then a Democratic Congress will make this a priority in January.”
Representative Adam B. Schiff, Democrat of California, who could become the new chairman of the House Intelligence Committee, said that any interference with the Mueller investigation “would cause a constitutional crisis and undermine the rule of law.”
But Republicans in Congress appeared less concerned by the president’s move. Senator Lindsey Graham, Republican of South Carolina, who said in 2017 that there would be “holy hell to pay” if Mr. Trump fired his attorney general, offered no criticism of the president on Wednesday.
“I look forward to working with President Trump to find a confirmable, worthy successor so that we can start a new chapter at the Department of Justice,” Mr. Graham said. He had in recent months begun to ease off his stance of last year, saying in August that it had become clear that Mr. Sessions had lost the president’s confidence.
The firing of Mr. Sessions came a day after midterm elections that handed control of the House to Democrats, dealing a major blow to Mr. Trump for the final two years of his term. Republicans preserved their hold on the Senate and increased their majority slightly, making it likelier that Mr. Trump would be able to confirm a replacement.