Congressional Democrats are warning President Trump not to interfere with special counsel Robert Mueller’s probe into Russian meddling in the 2016 election after a federal grand jury indicted former Trump campaign chairman Paul Manafort and his former business partner Rick Gates.

The special counsel’s office announced Monday that Manafort and Gates were indicted on 12 counts, including conspiracy to launder money and conspiracy against the United States. Both Manafort and Gates surrendered to federal authorities Monday morning and pleaded not guilty to all charges in court Monday afternoon.

The indictment’s unsealing prompted Democrats to speak of the need for continued independence for Mueller and caution Trump against firing the special counsel.

“The reported indictments show that the special counsel’s probe is ongoing in a very serious way. The rule of law is paramount and the investigation must be allowed to continue unimpeded,” Senate Minority Leader Chuck Schumer, D-N.Y., said in a statement. “The president must not under any circumstances interfere with the special counsel’s work in any way.”

Sen. Bernie Sanders, I-Vt., echoed Schumer’s calls to ensure the president doesn’t hinder Mueller’s investigation.

“Russia’s interference in our recent election and their attack on American democracy is an issue of enormous consequence,” Sanders said. “Special Counsel Mueller, appointed with bipartisan support, is proceeding with his investigation into the relationship between the Trump campaign and Russian interests. President Trump must not, in any way, try to derail or obstruct this effort.”

Sanders caucuses with Senate Democrats despite being elected as an independent and sought the party's presidential nomination last year.

Manafort and Gates are the first two people to be charged in Mueller’s investigation. But in addition to their indictment, the special counsel’s office also unsealed a case Monday involving George Papadopoulos, a former foreign policy adviser for the Trump campaign.

Papadopoulos pleaded guilty to lying to FBI agents about his contacts with Russians during an interview in January.

Trump frequently voices his displeasure with Mueller’s investigation, calling it a “witch hunt” and asserting his campaign didn’t collude with Russia.

Some congressional Republicans, too, have questioned Mueller’s ability to pursue an unbiased investigation given his closeness to former FBI Director Jim Comey, who Trump fired in May. Others have called for Mueller to step down.

But White House press secretary Sarah Sanders told reporters Monday “there’s no intention or plan to make any changes in regard to the special counsel.”

Still, Democrats are ramping up the pressure for their Republican counterparts to work alongside them to ensure Mueller will be shielded from any political interference.

“Today’s actions by the Special Counsel may prompt the White House and its allies to push back even more strongly — by seeking to curtail the congressional investigations or the Special Counsel’s work prematurely, by shifting the focus to investigating the President’s former opponent, or by other means,” Rep. Adam Schiff of California, the top Democrat on the House Intelligence Committee said. “Whatever the response, Congress — Democrats and Republicans alike — must make clear that we as an institution will do everything in our power to protect the Special Counsel’s independence.”

Sen. Dianne Feinstein of California, the ranking member of the Senate Judiciary Committee, said it’s imperative for Mueller’s independence to be protected.

“Today’s charges against Paul Manafort and Rick Gates, two integral members of President Trump’s campaign, show that the special counsel is doing his job and the process is working,” Feinstein said. “I’ll continue to support Bob Mueller as he follows the facts—his independence must remain sacrosanct.”

Many Republicans were quiet after the special counsel announced the indictment against Manafort and Gates.

But at least two GOP lawmakers, Rep. Jim Banks of Indiana, and Senate Judiciary Committee Chairman Chuck Grassley, R-Iowa, spoke of the importance of ensuring Mueller’s probe moves forward.

“Months ago I & many other Republicans vowed to support Mueller investigation & allow it to work its way through process to get the facts,” Banks tweeted Monday. “In light of today’s indictments we must continue to support and allow the integrity of the process to work.”

Grassley said Trump should refrain from interfering with the investigation.

“The president should let the special counsel do his job,” he told reporters.

In an effort to provide an extra layer of protection for Mueller, lawmakers introduced bipartisan bills earlier this year intended to make it more difficult for the president to fire the special counsel.

One, introduced by Sens. Thom Tillis, R-N.C., and Chris Coons, D-Del., would allow a special counsel who is removed to challenge his or her firing in court, where a three-judge panel would decide whether the firing was justified.

Another, introduced by Sens. Lindsey Graham, R-S.C., Cory Booker, D-N.J., Sheldon Whitehouse, D-R.I., and Richard Blumenthal, D-Conn., would require a Justice Department official to receive approval before firing a special counsel.