Michael Cohen, President Donald Trump's former personal lawyer, pleaded guilty Thursday in federal court in New York to lying to Congress about plans to build a Trump Tower in Russia.

The plea marked a major new development in Justice Department special counsel Robert Mueller's investigation into Russian meddling in the 2016 election. Court documents filed as part of the plea deal detailed Trump's business dealings in Russia that he long minimized and that lasted longer during his campaign than previously acknowledged.

Trump on Thursday called Cohen "a weak person" and said even if his testimony were true, it was perfectly legal for him to negotiate a real-estate deal while campaigning.

“He's a weak person and not a very smart person," Trump said. "Even if he was right, it doesn't matter because I was allowed to do whatever I wanted during the campaign. I was running my business."

The guilty plea came amid a flurry of activity from Mueller, who personally signed Cohen's deal. On Monday, Mueller voided a plea deal with Paul Manafort, Trump's former campaign chairman, for lying repeatedly to investigators. Manafort has a hearing Friday to get a possible sentencing date for charges of conspiracy and obstruction of justice for representing a pro-Russia faction in Ukraine.

Other Trump supporters have been scrutinized about how much they knew about a dump of Democratic nominee Hillary Clinton's campaign emails that Mueller has charged were hacked by Russian military intelligence officers.

Roger Stone, a longtime Trump adviser and Republican political operative, has said he could be indicted even as he protested he did nothing illegal. One of his associates, Jerome Corsi, a peddler of conspiracy theories, said Monday he rejected a plea deal from Mueller.

Cohen pleaded guilty to a single count of lying to Congress about a Trump Tower development project in Moscow, according to the U.S. Attorney's Office in Manhattan.

Federal prosecutors said Cohen lied when he submitted an Aug. 28, 2017, letter to the Senate and House intelligence committees. The letter said the project had ended by January 2016, when planning continued months longer during the presidential campaign.

Prosecutors said that Cohen lied to the committees to “minimize links between the Moscow Project and (Trump) and give the false impression that the Moscow Project ended before the Iowa caucus and the very first primary in hopes of limiting the ongoing Russia investigations.”

Cohen also pleaded guilty in August to eight counts related to providing hush money to two women who claimed to have had sex with Trump. But that case was handled by federal prosecutors in the Southern District of New York, rather than by Mueller's team.

According to the plea agreement, Cohen has spent months in discussions with Mueller’s team, beginning in August and continuing through last week.

Jens David Ohlin, a criminal-law expert who is vice dean of Cornell Law School, said Cohen's plea offers a motive for Trump's repeated deference to Russia.

"The motive is money and business deals," Ohlin said. "This gives Mueller the last piece of the puzzle.”

Cohen faced a maximum of five years in prison and a $250,000 fine for lying to Congress, but the plea agreement called for a sentence up to six months and a fine up to $9,500.

Lanny Davis, an attorney for Cohen, tweeted Thursday that the plea illustrated that Mueller thought Cohen was telling the truth and that Cohen would continue cooperating with the special counsel.

Thursday's court appearance came after Cohen had long sought to shield the Trump Organization’s involvement in the Moscow project throughout the campaign, and while Trump has repeatedly denied that he had any business interests in Moscow in the run-up to the election.

Cohen’s guilty plea centers on false statements he made to the committees, asserting that discussions related to the Moscow project had ended in January 2016. In fact, according to court documents, the project discussions continued well into Trump’s presidential campaign.

“As late as approximately June 2016, Cohen… discussed the status and progress of the Moscow Project with (Trump) on more than the three occasions," according to a court filing.

A few hours before Cohen's plea, Trump again used Twitter to blast Mueller's investigation as politically motivated.

"When will this illegal Joseph McCarthy style Witch Hunt, one that has shattered so many innocent lives, ever end-or will it just go on forever?" Trump said.

Trump left Thursday morning for a Group of 20 nations summit in Buenos Aires, Argentina. He had planned to meet Saturday with Russian President Vladimir Putin at that conference but tweeted Thursday that he would cancel the meeting.

In the midst of the campaign, while Cohen was still at work on the proposed Moscow Trump Tower project, Trump emphatically denied any such interests in an interview with a Florida television station.

“I mean I have nothing to do with Russia," Trump said. "I don’t have any jobs in Russia. I’m all over the world, but we’re not involved in Russia.”

But Trump first traveled to Moscow in the 1980s, to discuss renovating hotels there. After several bankruptcies made it hard to raise money in the United States for his high-end hotel and condominium projects since the 1990s, Trump, and later his children, traveled to Moscow to talk deals and attract buyers, according to interviews with people who have worked with Trump over the years and news accounts.

Congressional Democrats plan to investigate Cohen's revelations. Sen. Mark Warner, D-Va., vice chairman of the Senate Intelligence Committee, said Cohen’s guilty plea is evidence that Trump associates sought to hide their dealings with Moscow during the campaign.

“The characters that have been affiliated with the Trump campaign have a history of not having much connection to the truth and that also comes with the White House that on a daily basis seems to have a disconnect from the truth,” Warner said.

Rep. Adam Schiff, D-Calif., who is poised to become chairman of the House Intelligence Committee when Democrats take control of the chamber in January, said the guilty plea demonstrated that Trump’s associates “were willing to lie to Congress about the Trump Organization’s interests in Russia” and that the president’s own statements were misleading.

“These false statements regarding the continued pursuit of a Moscow Trump Tower deal during much of the presidential campaign only underscore the importance of a thorough investigation into any financial entanglement between Trump and Russia,” Schiff said. “Our committee, and the new House majority, will use all of its powers to preserve the independence of our justice system and ensure that no one is above the law.”

Several legal analysts pointed out that Mueller appears to have stepped up legal activity after Trump provided written answers Nov. 20 to questions from the special counsel.

It "feels like Mueller was waiting for Trump to turn his answers in before he moved to the next phase," tweeted Matthew Miller, a spokesman for the Justice Department during President Barack Obama's administration.

"And now that he has them, the other shoes are starting to drop," Miller added.

The advocacy group Citizens for Responsibility and Ethics in Washington tweeted that Cohen's guilty plea is "a big deal."

"The president's lawyer is pleading guilty to lying about work on a business deal to put a Trump-branded property up in Moscow," the group tweeted. "No wonder the president is so mad about the Mueller investigation this morning."

Contributing: Jessica Estepa, Nicole Gaudiano, Eliza Collins and Kevin McCoy

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