WASHINGTON — Congressional Democrats said Friday that the arrest and indictment earlier in the day of former Trump adviser Roger Stone in the Russia probe could foreshadow even bigger developments.

“Roger Stone, Paul Manafort, Michael Cohen, Rick Gates, Michael Flynn... What did the President know and when did he know it?” tweeted Rep. Jerry Nadler, D-N.Y., chairman of the House Judiciary Committee.

Stone, 66, was arrested by FBI agents early Friday morning in Fort Lauderdale, Florida, and has been indicted by a federal grand jury through Robert Mueller’s investigation on charges of obstruction, giving false statements and witness tampering.

Some Democrats focused on the threat to Mueller's probe. "It is incumbent, it is imperative that all of us, whether we're senators or whether we’re citizens at home do everything we can to defend the Mueller investigation. Think that's what we need," Sen. Michael Bennet, D-Colo., said on MSNBC's "Morning Joe."

Those members on panels investigating Russian interference zeroed in on Stone because of his alleged connection to WikiLeaks and hacked Democratic emails released by the site during the 2016 presidential campaign. He has repeatedly denied any collusion with WikiLeaks.

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“I expect that we will learn more about Mr. Stone’s campaign role, his communications regarding Wikileaks, and who else knew about Stone’s efforts. It remains essential that the special counsel be permitted to finish this work without any political interference," said Sen. Mark Warner, D-Va., the ranking member of the Senate Intelligence Committee.

House Intelligence Committee Chairman Adam Schiff, D-Calif., said the Stone indictment "provides new and important detail" about the Trump campaign's actions.

"Our Committee will be eager to learn just who directed a senior campaign official to contact Stone about additional damaging information held by Wikileaks, one of the publishing arms of Russian government hackers," he said in a statement. "The first order of business for the Committee will be to release all remaining transcripts to the Special Counsel’s Office, and we will continue to follow the facts wherever they lead.”

While many leading Republicans were silent on the arrest, several Democrats mentioned the possible legal threat to Trump and others in his orbit.

Sen. Chris Coons, D-Del., said in an interview on "Morning Joe" that there are only two options: either Trump himself was actively involved in collusion, or he surrounded himself on the campaign with a circle of people who have now been charged with engaging in criminal activity, and that the "witch hunt" theory he has plugged is now disproven.

"Since WikiLeaks was the vehicle through which a vast store of emails and materials that were damaging to the Democratic campaign and to the DNC and to the Democratic nominee were released to the public, this strongly suggests that this might be the connection point between the Trump Campaign’s leadership and WikiLeaks and Russian Intelligence," Coons said. "This strongly suggests there was exactly that connection between the senior most levels of the Trump campaign."

Sen. Richard Blumenthal, D-Conn., a member of the Senate Intelligence Committee, which is also conducting its own probe, said the indictment could be a sign of things to come. “Stone indictment a possible preview of coming attractions — in living color, more dynamite disclosures are likely from this dramatic Trump crony & potential co-conspirator,” he tweeted. “Stone is an existential threat to Trump. A claimed key conduit for Russian communication, Stone was as close to Trump as anyone in the campaign — with possibly damning info on him & others.”

Others went even further. “Stone indictment clears it up: the Trump organization and campaign and administration are criminal enterprises,” said Rep. Gerry Connolly, D-Va.

“Who directs a senior “Trump Campaign Official?” ONLY a more senior campaign official. #RogerStone,” tweeted Rep. Eric Swalwell, D-Calif., another member of the House Intelligence panel and possible 2020 Democratic presidential contender.

Speaker Nancy Pelosi, D-Calif., gave a measured response, saying it was "very interesting to see the kinds of people that the president of the United States has surrounded himself with."

"This connection to the integrity of our elections is obviously something we have to get to the truth about," she said as she arrived at the Capitol Friday. "But it also bothersome to see his connections to Russia and the president’s suggestions that we should question whether we should be in NATO which is a dream come true for Vladimir Putin."

Leading Republicans were mostly silent on the arrest. Rep. David Cicilline, D-R.I., noted that the FBI agents who arrested Stone on Friday are not being paid because of the partial government shutdown, which is now in its 35th day.

“That’s how much of a bad guy he is. They went out and got him for free," he tweeted. "Now let’s end this shutdown so these heroes and other working people get their paychecks!”