Senator from California, Kamala Harris, addresses the media January, 21 2019 at Howard University in Washington, DC after announcing earlier in the day that she is seeking to become the first African American woman to hold the office of US president, joining an already-crowded field of Democrats lining up to take on Donald Trump.

Democrats seeking their party's nomination for president tread lightly on Friday after President Donald Trump's longtime confidant Roger Stone was indicted by special counsel Robert Mueller on charges of lying to Congress and witness tampering.

The charges were the first brought by Mueller since the Democratic primary kicked off in earnest at the start of the year with a slate of new entrants announcing either exploratory committees or formal bids for the nomination. Mueller, a former FBI director, is investigating Russian interference in the 2016 election.

In the first hours after the indictment was unsealed, candidates remained silent on the matter.

Three candidates, Sen. Kamala Harris of California, John Delaney, a former U.S. representative for Maryland, and Sen. Kirsten Gillibrand of New York issued statements calling for the special counsel to proceed after receiving requests for comment from CNBC. Trump has said the investigation should be halted.

"This morning's news is yet another sign that Special Counsel Mueller must be allowed to complete his investigation free of interference - and his report must be made public," Harris wrote in a post on Twitter from her Senate account, @SenKamalaHarris.

Her tweet came shortly after her campaign received a request for comment from CNBC, which it did not respond to. Shortly after that tweet, her campaign account, @KamalaHarris, called for legislation to be "brought to the Senate floor immediately to protect his investigation." That plea was repeated on her Facebook page.

Delaney's press secretary, Michael Starr Hopkins, told CNBC in a statement that the indictment is the "latest evidence that we must allow the Independent Counsel's investigation to continue."

"As my mom has always said, tell me who your friends are and it will tell me who you are," he said.

In 2016, Hopkins served on Hillary Clinton's voter protection team in Colorado.

"This is another deeply troubling indictment," Sen. Kirsten Gillibrand of New York said in a statement after this story was originally published. "Special Counsel Mueller's investigation is critical to our democracy, and he must be allowed to complete his work unimpeded, so that he can present all of the facts to Congress and the American people. Then we can make a decision on how to proceed."

The indictment comes as most Democratic contenders are visiting the early states of Iowa, New Hampshire, South Carolina and Nevada. They have largely refrained from taking the president head on as they seek to introduce themselves to the country.

Online, the candidates have been somewhat more direct, with both Harris and Gillibrand running ads Friday on Facebook directly targeting the president. Those ads are not related to the Stone indictment, which does not mention Trump but does refer to senior officials on his campaign. CNBC identified one of those officials as Steve Bannon.

Sen. Elizabeth Warren, D-Mass. did not respond to a requests for comment from CNBC on Friday.

The relative quiet from the candidates on Friday was in contrast to comments made by those who worked for Clinton in 2016, as well as some senior lawmakers.

Emily Bengtson, a Gillibrand staffer and former social media operative for Clinton's 2016 campaign, wrote in a post on Twitter following the indictment that Clinton "tried to tell you."

"[She] was right about everything, and all it would have taken to stop all of this was just listening to the woman," Bengtson wrote.

Phillip Reines, one of Clinton's longtime aides who played the role of Trump during her mock debate practice sessions in 2016, said in a tweet that Stone was a "gutless traitor."

John Podesta, Clinton's former campaign chairman, said in a post that it was "Rogers's time in the barrel—and it's headed over Niagara Falls." (In August 2016, two months before WikiLeaks published Podesta's stolen emails, Stone tweeted that it would soon be "Podesta's time in the barrel.")

Rep. Adam Schiff, D-Calif., the chairman of the House Intelligence Committee, said in a statement that the "indictment of Roger Stone, released today by the Special Counsel, provides new and important detail about the Trump campaign's effort to acquire dirt on Hillary Clinton."

The special counsel's counterintelligence investigation into Russian hacking is expected to take on new significance headed into the 2020 race as law enforcement seeks to prevent foreign actors from interfering in a second consecutive presidential election.

The U.S. intelligence community has accused the Kremlin of interfering in the race in favor of Trump, though the president has questioned that assessment.

The White House on Friday said the charges brought against Stone have no bearing on the president or his administration. In a tweet, the president repeated his claim that the Russia probe is a "witch hunt," and suggested without evidence that law enforcement had leaked news of Stone's arrest to CNN, one of Trump's frequent targets, in advance.

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CNN said its reporters were not tipped off ahead of time.

Attorneys for Stone said Friday they plan to vigorously contest the charges. Stone said he would plead not guilty.