“Today’s announcement has nothing to do with the president, has nothing to do with the president’s campaign or campaign activity,” said Sarah Huckabee Sanders, the White House press secretary. She added, “We’ve been saying from Day 1 there has been no evidence of Trump-Russia collusion, and nothing in the indictment today changes that at all.”

Jay Sekulow, a private lawyer for Mr. Trump, said the president and his legal team were not worried about the indictments. “No, not concerned,” Mr. Sekulow said on CNN. “I’m completely convinced, as I was from the outset, that not only was there no Russian collusion, there was no obstruction.” He added, “I’m not concerned about this at all, and no one else is either.”

But lawyers and former prosecutors said Mr. Papadopoulos’s admissions and the previously reported meeting involving Donald Trump Jr. already undercut such denials.

“Collusion is what Papadopoulos did. Collusion is what Trump Jr. and others in that meeting did,” Mr. Barrett said. “It’s meeting and discussing and seeing what common interests they can advance for each other.”

Mr. Mueller’s action also made it harder for Mr. Trump to brush off the investigation and blame Democrats. “After Mr. Trump whipped up a tweet storm of suspicion about Mueller this weekend, he really now has no place to go with this attack,” said Robert F. Bauer, a White House counsel under President Barack Obama. “Mueller’s first charge is beyond any potential claim of ‘politics’ or ‘stretching’ that the president might wish to bring against him and his office.”