WASHINGTON — Donald J. Trump was more involved in discussions over a potential Russian business deal during the presidential campaign than previously known, his former lawyer Michael D. Cohen said Thursday in pleading guilty to lying to Congress. Mr. Trump’s associates pursued the project as the Kremlin was escalating its election sabotage effort meant to help him win the presidency.
Mr. Trump’s participation in discussions about building a grand skyscraper in Moscow showed how the interests of his business empire were enmeshed with his political ambitions as he was closing in on the Republican nomination for president. During the early months of 2016, when the business discussions were taking place, he was publicly pressing for warmer relations between the United States and Russia and an end to economic sanctions imposed by the Obama administration, policy positions that might have benefited his family business.
Court documents made public by the special counsel, Robert S. Mueller III, detailed new accusations against Mr. Cohen, the president’s former fixer, who already pleaded guilty this year to committing campaign finance violations and financial crimes. Mr. Cohen was the point person at the Trump Organization for negotiating a deal for the Moscow project, and on Thursday he admitted lying to congressional investigators about the duration of the negotiations and the extent of the involvement of Mr. Trump — who is identified in the court documents as “Individual 1.”
After pleading guilty in a Manhattan courtroom on Thursday morning, Mr. Cohen said that he made the false statements to Congress out of loyalty to the president and to align with Mr. Trump’s “political messaging.”
Mr. Cohen’s cooperation with the special counsel’s investigation raises the possibility that he might have information about the central focus of the inquiry: whether President Trump or any of his associates conspired with Russia’s efforts to disrupt the 2016 election. And it was the second time that Mr. Cohen has imperiled the presidency; he said in court in New York in August that Mr. Trump directed hush money payments during the 2016 campaign to conceal potential sex scandals.
The Trump Tower discussions were occurring as Russia ramped up its sabotage campaign, the information provided on Thursday by Mr. Cohen showed, though the documents do not say whether any of Mr. Trump’s advisers were aware of the Russian disruption effort. According to a grand jury indictment made public this year, Russian intelligence operatives hacked the emails of Hillary Clinton’s campaign chairman in March 2016. That same month, an obscure professor whom Mr. Mueller’s team has identified as a likely cutout for Russian intelligence began courting a Trump campaign aide, George Papadopoulos.
Mr. Cohen admitted that the discussions about Trump Tower Moscow went on for at least six months after he had told Congress they had ended. They lasted until at least June 14, 2016, when Mr. Cohen met in New York with an associate who had been trying to arrange his trip to Russia, and told him he would not be traveling “at that time,” court documents said. Mr. Cohen also discussed the deal in a 20-minute phone call with a Russian government employee.
That same day, The Washington Post reported that Russian operatives had infiltrated the computer network of the Democratic National Committee — the first public evidence of Moscow’s campaign to disrupt the election.
Mr. Cohen said on Thursday that he discussed the status of the project with Mr. Trump on more than the three occasions he had previously acknowledged and briefed Mr. Trump’s family members about it.
Some of those exchanges, which continued until January 2016, included the president’s children Ivanka Trump and Donald Trump Jr., both of whom are executives at the Trump Organization, according to three people familiar with the documents that the company turned over to Mr. Mueller’s team.
In one email exchange in 2015, Ms. Trump made a suggestion about the architecture, according to two of the people familiar with the messages.
Donald Trump Jr. appeared to have replied to only one message, saying “Cool” in response to an update about the project, the people said.
Mr. Trump and Mr. Cohen discussed Mr. Trump himself traveling to Russia after the Republican National Convention, though that trip never materialized.
Mr. Trump defended his role in the Trump Tower Moscow discussions, brushing aside concerns that he was advancing his business interests at the time he was hoping to become president.
“There was a good chance that I wouldn’t have won, in which case I would have gotten back into the business, and why should I lose lots of opportunities?” he said to reporters as he left Washington for the Group of 20 meeting in Buenos Aires.
“We decided — I decided ultimately — not to do it,” he said, adding, “There would have been nothing wrong if I did do it.”
Mr. Trump accused his former fixer of lying to receive a reduced sentence for the crimes he has pleaded guilty to. Under his earlier plea agreement, Mr. Cohen faced about four to five years in prison.
“He was convicted of various things unrelated to us,” Mr. Trump said, adding, “He’s a weak person and what he’s trying to do is get a reduced sentence.”
The proceedings in Lower Manhattan appeared to have global repercussions. After Mr. Cohen’s appearance in court, Mr. Trump abruptly canceled a planned meeting with President Vladimir V. Putin of Russia while both leaders are in Argentina. The president said he called off the meeting because of Russia’s recent hostilities with Ukraine.
The new revelations were certain to increase the strain between Mr. Trump and the Justice Department. In recent days, the president and his lawyers have increased their attacks on the department and the Russia investigation, including Mr. Mueller. This week, the special counsel’s office accused Mr. Trump’s onetime campaign chairman, Paul Manafort, of repeatedly lying to investigators in breach of his plea agreement.
The acting attorney general, Matthew G. Whitaker, was told of Mr. Cohen’s impending plea Monday or earlier, a person familiar with the special counsel’s investigation said. The White House learned of it late Wednesday, people close to Mr. Trump said.
The new revelations also came a week after Mr. Trump’s lawyers provided Mr. Mueller with written responses to a set of questions.
The special counsel identified Mr. Cohen’s false statements to Congress in testimony and materials that Mr. Cohen provided to the Senate and House Intelligence Committees, which have been conducting their own investigations into possible coordination between the Trump campaign and Russia.
“There’s a reason people shouldn’t lie when they’re in front of a congressional investigation,” Senator Richard M. Burr of North Carolina, the chairman of the Intelligence Committee, said on Thursday. The committee’s top Democrat, Senator Mark Warner of Virginia, said that the panel had made additional criminal referrals to Mr. Mueller, but he gave no specifics.
Mr. Trump said on Thursday that discussions about the Moscow project took place in early 2016, but that he did not know exactly when he decided to stop pursuing it.
The president has previously said that there would be no problem if he continued to run his business from the White House. “I have a no-conflict-of-interest provision as president,” Mr. Trump said in January 2017, adding, “I have no deals that could happen in Russia, because we’ve stayed away.”
Mr. Cohen’s new guilty plea in Federal District Court is the first time the office of the special counsel has charged him. In exchange for pleading guilty and continuing to cooperate with Mr. Mueller, he may hope to receive a lighter sentence.
He is scheduled to be sentenced in two weeks. It is unclear what additional time he could face with the new guilty plea.
During his plea hearing in August, Mr. Cohen admitted to making a $130,000 payment to an adult film actress, Stephanie Clifford, better known as Stormy Daniels, to keep her quiet about an affair she said she had with Mr. Trump.
The payment amounted to an illegal contribution to Mr. Trump’s campaign, prosecutors argued, since her silence bolstered his election hopes and campaign finance law prohibits individuals from donating more than $2,700 to a presidential candidate.
He said that the payments to Ms. Clifford and Karen McDougal, a former Playboy model, were made “in coordination with, and at the direction of, a candidate for federal office” — a reference to Mr. Trump.
Representative Adam B. Schiff of California, the top Democrat on the House Intelligence Committee, said Mr. Cohen’s plea on Thursday only added urgency for congressional investigators who plan to begin an inquiry into Mr. Trump’s foreign business dealings when Democrats take control of the House in January.
“If the president and his associates were being untruthful in real time as they were pursuing this deal,” Mr. Schiff told reporters on Thursday, “what does it mean now on how much we can rely on what the president is saying about any continuing Russian financial interest?”