Robert S. Mueller III’s statement on Wednesday, in which he reiterated the conclusions of his investigation and declined to clear President Trump, seemed to open a dam in the Democratic presidential field.
Before Mr. Mueller spoke, seven of the 23 candidates had endorsed impeachment proceedings against Mr. Trump. By midafternoon, 10 had done so, with others edging closer.
Senator Cory Booker of New Jersey, Mayor Pete Buttigieg of South Bend, Ind., and Senator Kirsten Gillibrand of New York came out explicitly in favor of impeachment on Wednesday. They joined Senators Kamala Harris of California and Elizabeth Warren of Massachusetts, Representatives Seth Moulton of Massachusetts and Eric Swalwell of California, the former housing secretary Julián Castro, former Representative Beto O’Rourke of Texas and Mayor Wayne Messam of Miramar, Fla.
Representative Tim Ryan of Ohio called on Congress to “get to the bottom of this and unpack it with full transparency,” implying impeachment proceedings but not saying the word. Senator Bernie Sanders of Vermont, too, avoided an outright endorsement but tweeted, “If the House Judiciary Committee deems it necessary, I will support their decision.” Senator Amy Klobuchar of Minnesota said Congress had a mandate to investigate the administration and hold it accountable, and that “impeachment proceedings are one way” to do that.
A representative for former Vice President Joseph R. Biden Jr. said in a statement that Mr. 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.”
And former Representative John Delaney of Maryland, retweeting a noncommittal statement from Speaker Nancy Pelosi, wrote that he trusted her to find “the best path forward to hold the President accountable.”
Other candidates stopped shorter. Mayor Bill de Blasio of New York tweeted only that Congress “must continue to investigate” Mr. Trump, and that he was “focused on beating @realDonaldTrump in 2020.” Gov. Steve Bullock of Montana wrote that “Congress should fulfill its constitutionally empowered oversight of this Administration.” Gov. Jay Inslee of Washington tweeted, “Robert Mueller made it clear: No exoneration,” but said nothing about what Congress should or should not do.
The ground has shifted since the Justice Department released its redacted version of Mr. Mueller’s report. While a few candidates, including Ms. Warren, called for impeachment then, they were in the minority. Now, a majority of the Democratic field has expressed support, explicitly or implicitly, for impeachment.
Last month, Mr. Booker said he would reserve judgment until Congress received a full version of the report, telling voters in Nevada, “There’s a lot more investigation that should go on before Congress comes to any conclusions like that.” On Wednesday, however, he said on Twitter that legislators had “a legal and moral obligation” to pursue impeachment.
Mr. Buttigieg tweeted that Mr. Mueller “could not clear the president, nor could he charge him — so he has handed the matter to Congress, which alone can act to deliver due process and accountability.” Ms. Gillibrand wrote similarly that after listening to Mr. Mueller’s statement, she believed he “clearly expects Congress to exercise its constitutional authority and take steps that he could not.”
Several candidates seized on one line in Mr. Mueller’s statement, in which he said that under Justice Department policy, filing criminal charges against Mr. Trump was never an option — but that the Constitution provided another mechanism. It was a clear reference to impeachment, and Mr. Buttigieg, Ms. Harris, Ms. Klobuchar and Ms. Warren all characterized it as, essentially, an impeachment referral.
In a lengthy email sent to supporters, Mr. Moulton, who this month had already called for impeachment, wrote: “The Mueller investigation has provided the evidence. It’s up to Congress to examine that evidence and pursue justice to its conclusion, whatever that may be, regardless of the political consequences.”
As sitting members of the House, Mr. Moulton and Mr. Swalwell — who tweeted pointedly on Wednesday, “Our founders gave us a checks & balances system” — are the only pro-impeachment candidates so far who actually have the power to affect the decision.
The other sitting representatives in the race are Tulsi Gabbard of Hawaii, who has said she opposes impeachment, and Mr. Ryan, who did not call for it explicitly on Wednesday but edged about as close to the line as he could without stepping across.
“The President, no President, is above the law,” Mr. Ryan wrote. “And it’s Congress’ job to make sure we are true to our founding principle that the President is not a King and must answer to the American people.”