WASHINGTON (Reuters) - Democratic presidential candidates ratcheted up the pressure on U.S. House Speaker Nancy Pelosi to start the process of impeaching President Donald Trump after Special Counsel Robert Mueller’s first public comments on his two-year Russia probe.

FILE PHOTO: Biden for President campaign buttons are seen for sale beside "Impeach Trump Now!" buttons as U.S. Democratic presidential candidate and former Vice President Joe Biden meets union workers at the Teamsters Local 249 hall during his first public event since announcing his bid for the 2020 Democratic presidential nomination in Pittsburgh, Pennsylvania, U.S. April 29, 2019. REUTERS/Aaron Josefczyk/File Photo

Mueller, who was assigned to investigate Moscow’s interference in the 2016 election, on Wednesday reaffirmed that his team did not consider filing charges against Trump for obstruction of justice because U.S. Justice Department policy bars the indictment of a sitting president.

Several Democratic candidates for the White House interpreted Mueller’s remarks as encouraging the Democratic-led House of Representatives to determine whether Trump tried to derail the probe and should be impeached.

“In essence, he said ‘We did not exonerate the president,’” U.S. Senator Kamala Harris of California said to loud applause at a campaign event in Greenville, South Carolina.

Pelosi has resisted calls from progressive members of her caucus to move forward on impeachment, arguing it could damage Democrats politically in advance of the November 2020 presidential election.

Speaking at an event in San Francisco, Pelosi urged caution, noting that only a minority of Democrats in the House have called for impeachment. “We want to do what is right and what gets results,” Pelosi said.

Still, after Mueller spoke, U.S. Senators Cory Booker of New Jersey and Kirsten Gillibrand of New York actively advocated impeachment for the first time.

Both previously were more restrained, criticizing the Trump administration for stonewalling congressional subpoenas and suggesting that continued obstruction could eventually lead to impeachment.

“Robert Mueller’s statement makes it clear: Congress has a legal and moral obligation to begin impeachment proceedings immediately,” Booker wrote on Twitter soon after Mueller finished speaking.

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Leading contenders such as Harris, U.S. Senator Elizabeth Warren of Massachusetts and former U.S. Congressman Beto O’Rourke of Texas had called for Trump’s impeachment prior to Mueller’s remarks.

O’Rourke on Wednesday tweeted that “there must be consequences, accountability, and justice. The only way to ensure that is to begin impeachment proceedings.”

Another top contender, South Bend, Indiana, Mayor Pete Buttigieg, had previously said Trump “deserves” to be impeached but said he would leave the decision to Congress.

On Wednesday, he told NBC News that Mueller’s statements were “as close to an impeachment referral as you could get under the circumstances.”

Former Vice President Joe Biden, the front-runner for the nomination, stuck to his more careful stance.

Biden “agrees with Speaker Pelosi that no one would relish what would certainly be a divisive impeachment process, but that it may be unavoidable if this administration continues on its path,” his campaign said in a statement.

Making sure Trump is not re-elected “is the sure-fire way to get him out of office,” the campaign said.

U.S. Senator Bernie Sanders, another leading contender who has not formally called for proceedings, said on Twitter that he would support a decision by the House Judiciary Committee to open an impeachment inquiry.

Overall, about a third of the 24 Democratic candidates have explicitly called on Congress to initiate impeachment proceedings. Several others either said they would follow the House’s lead or warned that continued Trump administration stonewalling could lead to impeachment as a last resort.

With Republicans in control of the U.S. Senate, which would likely refuse to convict Trump on charges brought by the House, some Democrats fear bringing impeachment proceedings would end up as a pointless exercise that could alienate moderate voters.

The Trump re-election campaign released a statement by campaign manager Brad Parscale that said Mueller’s remarks on Wednesday “fully and completely exonerated” Trump and that the “case is now closed.”