A footnote in Mr. Papadopoulos’s plea agreement includes a detail that is particularly damning when combined with previously reported information: Mr. Manafort wanted to be sure that Mr. Trump himself would not accept a Russian invitation to travel to Russia. In March 2016, George Papadopoulos sent an email to seven campaign officials, including Mr. Manafort and the campaign manager at the time, Corey Lewandowski, saying that Russian leadership wanted to meet with the Trump team. Mr. Manafort forwarded that email to Mr. Gates with a note saying: “We need someone to communicate that D.T. is not doing these trips. It should be someone low level in the campaign so as not to send any signal.”

Second, while Mr. Manafort and Mr. Gates’s Eastern European misadventures, beginning in 2005, were extensive and seamy, they have no immediate link to Mr. Trump’s campaign activities or even to his many financial adventures in Russia before he was a candidate. Connecting the dots to Mr. Trump’s own financial misdeeds — if there are any dots to connect — will be a long and laborious process. Not so with Mr. Papadopoulos.

Third, a paragraph in the plea agreement indicates that Mr. Papadopoulos pleaded guilty on Oct. 5 and the plea was sealed so that he could act as a “proactive cooperator.” The meaning of that phrase is unclear. But one nerve-racking possible implication is that Mr. Papadopoulos has recently worn a wire in conversations with other former campaign officials. This will surely have members of Mr. Trump’s inner circle agonizing about the possibility and wondering who else might have been similarly cooperating with the investigation.

Fourth, the plea agreement makes clear the Trump campaign knew about the hacking of Democratic National Committee emails well before it was publicly revealed. The email account of the chairman of Mrs. Clinton’s campaign, John Podesta, was hacked March 19, and Mr. Papadopoulos was approached with the offer of thousands of emails on April 26, at least a month before it was generally known and several months before WikiLeaks released any of the emails.

Fifth, the episode that prompts the guilty plea is a virtual carbon copy of the infamous June 9, 2016, meeting that Mr. Manafort, Jared Kushner and Donald Trump Jr. attended with a Russian lawyer. That meeting was set up through an email exchange between Mr. Trump Jr. and one of Donald Trump’s former business associates who told the younger Mr. Trump that a senior Russian government official had documents “that would incriminate Hillary” and that “would be very useful to your father.” Mr. Trump Jr.’s email response: “I love it.”