Senate Minority Leader Chuck Schumer, D-N.Y., said on Wednesday, moments after Attorney General Jeff Sessions resigned at President Donald Trump's request, that protecting special counsel Robert Mueller’s investigation is paramount.

“I find the timing very suspect, No. 1,” Schumer told reporters. “But No. 2, our paramount view is that any attorney general, whether this one or another one, should not be able to interfere with the Mueller investigation in any way."

"They should not be able to end it, they should not be able to limit it, they should not be able to interfere with Mueller going forward and doing what he thinks is the right thing. And that will help guide us as we go through this process," he said.

Schumer said any interference in the Mueller investigation would be considered a "constitutional crisis," and added that "we would hope, if there's a constitutional crisis, that our Republican colleagues would join us in thwarting the president from creating that crisis. We'll see. We've never known because, thank God, we haven't had it so far."

Senate Judiciary Committee Chairman Chuck Grassley, R-Iowa reacted to the news by expressing confidence in Whitaker's ability to lead the Justice Department.

"A Fellow Iowan, who I've known for many years, Matt will work hard and make us proud," Grassley said in a statement that made no mention of Mueller. "The Justice Department is in good hands during this time of transition."

At an earlier press conference in the Capitol before news of Sessions' departure broke, House Minority Leader Nancy Pelosi, D-Calif., spoke carefully about Democratic investigative plans, saying they had a duty to “honor our oversight responsibilities.”

Pelosi, poised to become speaker in January, did not directly answer questions on whether a Democratic House would pursue Trump’s tax returns, saying only that any investigations would be handled deliberately.

“I’m a big believer in the committee system. Always have been,” Pelosi told reporters. “Our committees will make their decisions and make their recommendations to the caucus. But you can be sure of one thing: When we go down any of these paths, we’ll know what we’re doing, and we’ll do it right.”

After the Sessions news, Pelosi took a tougher tone. "It is impossible to read Attorney General Sessions’ firing as anything other than another blatant attempt by @realDonaldTrump to undermine & end Special Counsel Mueller’s investigation," she tweeted.

She called for Matthew Whitaker, Sessions' chief of staff and Trump's pick for acting attorney general, to recuse himself from any involvement in the Mueller probe, "given his record of threats to undermine & weaken the Russia investigation."

The Trump loyalist has long been publicly critical of Mueller's probe, accusing investigators of overreach.

"Congress must take immediate action to protect the rule of law and integrity of the investigation," Pelosi said. She said in a statement that Democrats would continue to push for protections against the firing of the special counsel be inserted into an appropriations bill this fall.

Other top Democrats also expressed concern over Sessions' abrupt departure on Wednesday and the future of the Mueller investigation.

“There is no mistaking what this means, and what is at stake: this is a constitutionally perilous moment for our country and for the president,” Rep. Jerrold Nadler of New York, the ranking Democrat on the House Judiciary Committee, said in a statement Wednesday.

In the next Congress, Nadler is expected to chair the panel, which is positioned to play a key oversight role. “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," he said Wednesday.

Rep. Adam Schiff, D-Calif., who is likely to become the next chairman of the House Intelligence Committee, struck a similar note. "Interference with the Special Counsel's investigation would cause a constitutional crisis and undermine the rule of law," he said in a statement. "If the President seeks to interfere in the impartial administration of justice, the Congress must stop him. No one is above the law."

Sen. Jeff Merkley, D-Ore., offered a dire assessment. "This is how the cover-up begins," he tweeted. "This is a precarious moment, and Congress must immediately pass legislation to protect the work of the #MuellerInvestigation. The rule of law is at stake."

Merkley also called on Whitaker to immediately recuse himself from the Mueller investigation, tweeting that "he has no interest in overseeing this investigation in a way that is fair, impartial, and follows the rule of law."

Sen. Dianne Feinstein, D-Calif., the ranking Democrat on the Senate Judiciary Committee, insisted that Deputy Attorney General Rod Rosenstein continue to oversee the investigation as he had previously been doing after Sessions recused himself over a year ago.

Feinstein also urged Whitaker to testify before the Judiciary Committee "as soon as possible."

"No one who lacks Senate confirmation should be placed in charge of this investigation, especially not Matthew Whitaker who publicly criticized Robert Mueller's work just last year," Feinstein said in a statement.

Democrats have long pressed for legislation to protect Mueller's job, while Republican leadership has dismissed such measures as unnecessary.

On Wednesday, Retiring Sen. Jeff, Flake, R-Ariz., again called on Senate Majority Leader Mitch McConnell, R-Ky., to allow a bill protecting Mueller "to the Senate floor as soon as possible."

Senator-elect Mitt Romney, R-Utah, and Sen. Susan Collins, R-Maine, also broke from party leadership on Wednesday, raising concerns over the future of Mueller's investigation.

"It is imperative that the important work of the Justice Department continues, and that the Mueller investigation proceeds to its conclusion unimpeded," Romney tweeted.

Collins added, "Special Counsel Mueller must be allowed to complete his work without interference — regardless of who is AG."