WASHINGTON — A former top adviser to Donald J. Trump’s presidential campaign pleaded guilty on Friday to fraud and lying to investigators in the special counsel inquiry into Russia’s interference in the 2016 election and will cooperate with the investigation.
The adviser, Rick Gates, is a longtime political consultant who once served as Mr. Trump’s deputy campaign chairman. The plea deal could be a significant development in the investigation — a sign that Mr. Gates plans to offer incriminating information against his longtime associate and the former campaign chairman, Paul Manafort, or other members of the Trump campaign in exchange for a lighter punishment. He faces up to nearly six years in prison.
The deal came as the special counsel, Robert S. Mueller III, has been raising pressure on Mr. Gates and Mr. Manafort with dozens of new charges of money laundering and bank fraud that were unsealed on Thursday in Alexandria, Va. Both men were first indicted in October, and both pleaded not guilty.
Appearing with his attorney in a Washington courtroom on Friday afternoon, Mr. Gates changed his plea, acknowledging that he participated in the financial conspiracy with Mr. Manafort.
He also admitted that he lied to investigators earlier this month — after he was under indictment and was negotiating with the prosecutors — about the details of a 2013 meeting in Washington that Mr. Manafort had with a member of Congress and a lobbyist, during which there was a discussion about Ukraine, where Mr. Manafort and Mr. Gates worked as political consultants.

While the government outlined the charges against him, Mr. Gates grew red, his eyes cast downward. 
It is unclear exactly what he might have to offer the special counsel, or what his cooperation with Mr. Mueller means for President Trump. Neither indictment indicated that either Mr. Gates or Mr. Manafort had information about the central question of the investigation — whether Mr. Trump or his aides coordinated with the Russian government’s efforts to disrupt the 2016 election.
But Mr. Gates was present for the most significant periods of activity of the campaign, as Mr. Trump began developing policy positions and his digital operation engaged with millions of voters on platforms such as Facebook. Even after Mr. Manafort was fired by Mr. Trump in August 2016, Mr. Gates remained on in a different role, as a liaison between the campaign and the Republican National Committee. He traveled aboard the Trump plane through Election Day.
In addition to offering some visibility into the Trump campaign, Mr. Gates might be able to provide prosecutors with glimpses into decision-making in the months after the president’s election victory. He was a consultant on the transition team, and he also worked in 2017 with America First Policies, the main outside group supporting the Trump presidency.
Besides the plea agreement with Mr. Gates, the special counsel’s team has already secured guilty pleas from two of Mr. Trump’s advisers. Michael T. Flynn, the president’s first national security adviser, and George Papadopoulos, a foreign policy aide during the campaign, have both pleaded guilty to lying to the F.B.I. and agreed to cooperate with the inquiry.
Mr. Gates’s plea deal came together over the past few days, according to people familiar with the process. It was not immediately clear what the agreement will mean for the newly filed charges in Virginia; it is unusual for a new indictment to be brought on the eve of a plea change.
In a letter to friends and family, obtained first by ABC News, Mr. Gates said there had been false news stories about an impending plea deal over the past two weeks.
But, he added, “Despite my initial desire to vigorously defend myself, I have had a change of heart. The reality of how long this legal process will likely take, the cost, and the circuslike atmosphere of an anticipated trial are too much. I will better serve my family moving forward by exiting this process.”
If Mr. Manafort continues to fight the charges in a trial, testimony from Mr. Gates could give Mr. Mueller’s team a first-person account of the criminal conduct that is claimed in the indictments — a potential blow to Mr. Manafort’s defense strategy.
In a statement on Friday, Mr. Manafort said, “Notwithstanding that Rick Gates pleaded today, I continue to maintain my innocence. I had hoped and expected my business colleague would have had the strength to continue the battle to prove our innocence. For reasons yet to surface he chose to do otherwise. This does not alter my commitment to defend myself against the untrue piled up charges contained in the indictments against me.”
The court papers give few specifics about how Mr. Gates came to be charged with lying to the F.B.I. On Feb. 1, according to the records, Mr. Gates misled investigators about a conversation he had with Mr. Manafort in March 2013, after Mr. Manafort had met with a congressman discuss the situation in Ukraine. The documents do not name the lawmaker, but press accounts have identified him as Representative Dana Rohrabacher of California, the Republican long known for his pro-Russia views.
Mr. Gates falsely told investigators that Mr. Manafort had told him that the subject of Ukraine had not come up at the meeting, even though Mr. Gates had helped draft a report to Ukraine’s leadership after the meeting about what had transpired, according to the court papers.
They detail a byzantine scheme he and Mr. Manafort used from about 2006 to 2015 where the two men funneled millions of dollars they earned from their work as political consultants in Ukraine into shell companies and foreign bank accounts. They then hid the existence of the companies and accounts, set up in Cyprus, Saint Vincent and the Grenadines and the Seychelles — from American tax authorities.
The men worked in various capacities with Viktor F. Yanukovych, the onetime Ukrainian president and a longtime ally of President Vladimir V. Putin of Russia.
Mr. Mueller’s team found that more than $75 million passed through offshore accounts, and that Mr. Manafort laundered more than $18 million to furnish a lavish, largely tax-free lifestyle. Mr. Gates transferred more than $3 million from the offshore accounts, court documents show.
Mr. Manafort purchased multimillion-dollar homes, expensive clothing and other luxury goods. Mr. Gates used the money to pay his mortgage, school tuitions, and for interior decorating of his home in Virginia, the documents show.
The work the two men did for their firm, Davis Manafort, connected them to numerous people with ties to the Kremlin. One was Oleg Deripaska, an aluminum magnate and an ally of Mr. Putin. Mr. Deripaska has been denied a visa to travel to the United States because of allegations that he is linked to organized crime operations, claims he has denied.
In 2008, Mr. Gates took over the firm’s duties in Eastern Europe, where he worked on business development and contract negotiations.
After their work was disclosed in news reports in August 2016, when the two men were working for the Trump campaign, they “developed a false and misleading cover story” to distance themselves from Ukraine, according Mr. Mueller’s prosecutors.
And they covered their tracks when reporting their income to the Internal Revenue Service. Two months after Mr. Manafort left the campaign, according to the court documents, his accountant emailed him a question about whether he had any foreign bank accounts.