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Democrats said that Michael Flynn's guilty plea on Friday suggests there's a lot more to come in the special counsel's probe of the Trump campaign's ties to Russia, while Republicans reacted cautiously to the news.

"It is a shattering moment for the Trump presidency," Sen. Richard Blumenthal, D-Conn., told NBC News, a remark that was echoed by many Democrats who suggested the guilty plea was the start of something larger that could engulf the White House.

House Minority Leader Nancy Pelosi, D-Calif., said it was a "dark moment in our nation's history" and called for an investigation independent of the Justice Department and Congress.

"Republicans in Congress must stop shielding the President and join Democrats to take real, immediate action to counter Russian aggression and prevent further attacks on our democracy," she said.

Sen. Dianne Feinstein, D-Calif., the ranking member of the Judiciary Committee, said what Flynn "lied about and when he did it are of even greater significance."

"This shows a Trump associate negotiating with the Russians against U.S. policy and interests before Donald Trump took office and after it was announced that Russia had interfered in our election," she added. "That's a stunning revelation and could be a violation of the Logan Act, which forbids unauthorized U.S. citizens from negotiating with a foreign power."

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On MSNBC, Rep. Adam Schiff, D-Calif., said, "This is clearly not the last shoe to fall."

Sen. Mark Warner, D-Va., called the plea "part of an alarming pattern" and noted that it comes on the heels of a guilty plea by a campaign aide and charges against Paul Manafort, Trump's former campaign manager.

New Hampshire Democrat Sen. Maggie Hassan pressed for independence for the special counsel.

"It is critical that we get a full accounting of any national security threats that Flynn’s contact with Russia may have posed,” she said in a statement. "It is more important than ever that Special Counsel Mueller is given true independence to follow the facts wherever they may lead."

Republicans shied away from jumping to conclusions.

"As far as I understand, there's nothing here where he was colluding with the Russians," Sen. Lindsey Graham, R-S.C., said of Flynn.

Sen. Jim Inhofe, R-Okla., called the former national security adviser "a great American hero," and said, "I know him well enough to know that anything he does, he thought he was doing in the best interest of America, as his whole life has reflected."

But Sen. Shelley Moore Capito, R-W.Va., said "that's a pretty grave offense, lying to the FBI," adding, "I think that's troubling for him."

The Dow briefly dropped more than 300 points on news of the Flynn plea, before recovering later in the afternoon. The Dow closed down 41 points.

Prosecutors said Flynn, who pleaded guilty to one count of lying to federal investigators, was "directed" by an unnamed senior official with the presidential transition team and a "very senior transition official" in his dealings with the Russian ambassador. Two people familiar with the matter tell NBC News that Jared Kushner is the "very senior" transition official.

A source close to President Donald Trump said the developments regarding Flynn were "very, very, very bad."

Former FBI director James Comey, who has said Trump pressured him to drop his investigation into Flynn before he was fired, took to Instagram to respond to Flynn's guilty plea.

"But let justice roll down like waters and righteousness like an ever-flowing stream," Comey wrote in the caption of an Instagram post of a rocky river. "Amos 5:24."

CORRECTION (Dec. 1, 2017, 9:25 p.m., ET): An earlier version of this story misstated the party affiliation of New Hampshire Sen. Maggie Hassan. She is a Democrat, not a Republican.