Sen. Chuck Grassley Charles (Chuck) Ernest GrassleyGardner signals support for taking up Supreme Court nominee this year Grassley, Ernst pledge to 'evaluate' Trump's Supreme Court nominee McConnell digs in on vow to fill Ginsburg's Supreme Court seat MORE (R-Iowa) is urging President Trump to not interfere with special counsel Robert Mueller's investigation after Paul Manafort, Trump's former campaign chairman, was charged on Monday.

"The president should let the special counsel do his job," Grassley, the chairman of the Judiciary Committee, told reporters on Capitol Hill.

Senator Grassley reax to Mueller news: "all I know at this point is the president should let the special counsel do his job" — Alan He (@alanhe) October 30, 2017

Chuck Grassley just told me Trump shouldn't get in the way of Mueller: "The president should let the special counsel do his job." — Manu Raju (@mkraju) October 30, 2017

Grassley added in a separate statement that it's "important to let our legal system run its course."

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"While we don’t have any more information regarding the current status of the special counsel’s investigation other than what has already been made public, it’s good to see the Justice Department taking seriously its responsibility to enforce the Foreign Agents Registration Act (FARA)," he said.

Manafort has been charged with 12 counts including conspiracy against the United States, failing to register as a foreign agent and "false and misleading FARA statements," according to the special counsel's office.

Manafort’s former business partner and protegé, Rick Gates, who was ousted from the pro-Trump group America First Policies in April, has also been charged.

The charges are related to work done by Manafort and Gates on behalf of a pro-Russia political party in Ukraine.

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Grassley added, according to CBS News, that he is being briefed on the developments in the special counsel's investigation on Monday.

He noted in his statement that he has been working on legislation to improve the Justice Department's enforcement of FARA and will introduce it "very soon."

The escalation of Mueller's investigation comes as the Judiciary Committee's own wide-ranging investigation, which includes Russia's election interference, appears to have hit a setback.

Sen. Dianne Feinstein Dianne Emiel FeinsteinBiden leads Trump by 12 points among Catholic voters: poll Names to watch as Trump picks Ginsburg replacement on Supreme Court McConnell says Trump nominee to replace Ginsburg will get Senate vote MORE (D-Calif.) told reporters last week that she and Grassley "have decided that each side is going to take a course and we'll share and if the other side wants to participate they can. ... So we are going ahead with some things."

Feinstein last Friday sent a first tranche of letters as part of Democrats' new effort, which included questions about Manafort.

The split of what was a bipartisan probe appears to have arisen from the GOP push to revisit Obama-era scandals and dig into a controversial opposition research dossier compiled on Trump.

Grassley separately told reporters that he is still interested in looking at potential collusion with Russia.

—Updated at 2:50 p.m.