(CNN) President Donald Trump spoke more extensively during the presidential election with his-then attorney Michael Cohen about the proposed Trump Tower project in Moscow than Cohen admitted to Congress last year, Cohen said while pleading guilty Thursday in federal court to a charge from special counsel Robert Mueller's office.

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Cohen, who previously said talks about the Moscow project had ended in January 2016, just prior to the Iowa caucuses, said he had lied out of a sense of obligation to Trump.

"I made these statements to be consistent with Individual-1's political messaging and out of loyalty to Individual-1," Cohen said. Individual-1 was identified in court filings as Trump, and Cohen identified him as such in a New York courtroom Thursday.

Cohen, who famously once declared he would "take a bullet" for Trump, is cooperating with Mueller and has spoken with the special counsel's office for more than 70 hours on topics beyond Moscow, a source with knowledge of the discussions told CNN.

He pleaded guilty earlier this year to eight counts in a separate case from the Manhattan US attorney's office. Cohen did not have an agreement to cooperate with prosecutors on that case.

Thursday's revelations are potentially significant because they appear to show that Trump was engaged in business dealings with Russia in the midst of a campaign in which Moscow interfered to help elect him.

They could also intersect with other information that Mueller knows to create political and legal jeopardy for the President.

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Trump responded to Cohen's admission at the White House on Thursday, calling his former lawyer "very weak."

"He's a weak person," Trump said before departing for Buenos Aires, Argentina.

"He was convicted with a fairly long-term sentence with things unrelated to the Trump Organization," Trump said. "What he's trying to do is get a reduced sentence."

In fact, Cohen hasn't been sentenced in either case, and the charges to which he pleaded guilty in August included information about his reimbursement by the Trump Organization for payments he made or helped orchestrate to conceal allegations from two women about sexual encounters with Trump before he ran for office. Trump has denied those claims.

The Justice Department notified Trump's legal team on Wednesday night about Cohen's planned plea, a source familiar with the matter told CNN, although it is unclear when the President himself was informed.

Cohen was also charged in the Manhattan US attorney's office case with tax fraud and false statements to a bank. He is scheduled to be sentenced in both cases on December 12.

Cohen left the courthouse Thursday without making a statement.

Cooperating with Mueller

Cohen's cooperation with Mueller has included meetings with federal prosecutors on at least seven occasions beginning August 7, 2018, two weeks before he was first charged in New York, according to court filings.

Cohen had previously said talks about the Moscow project had ended in January 2016.

In a letter to Congress and in congressional testimony, Cohen had also stated that he never agreed to travel to Russia in connection with the project, and that he hadn't considered asking Trump to travel for the project. He had also said he didn't recall speaking to the Russian government about the project.

All of those statements were false, Cohen said Thursday.

"In truth and in fact, and as Cohen well knew, Cohen's representations about the Moscow Project he made to (House and Senate Intelligence committees) were false and misleading," Mueller's office said in a court filing.

"Cohen made the false statements to (1) minimize links between the Moscow Project and Individual 1 and (2) 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," prosecutors said in court filings.

Discussions with 'Individual-2' on Moscow project

The charges provide a much fuller picture of the Trump Organization's efforts to advance the project in Moscow.

As late as June 2016, according to prosecutors, Cohen and another man, identified in filings as "Individual-2," discussed efforts to gain Russian government approval for the Moscow project. Individual-2 is Felix Sater, a Russian-born onetime business associate of Trump's, according to people familiar with the matter.

Cohen discussed the project with Trump on more than the three occasions he had previously mentioned, prosecutors said, and he briefed Trump family members working within the Trump Organization about the efforts. Cohen also agreed to travel to Russia and asked Trump about the possibility of Trump going there in service of the project. In addition, Cohen "asked a senior campaign official about potential business travel to Russia," according to prosecutors.

In May 2016, after Sater asked Cohen when the trip involving Trump should occur, Cohen responded that Trump's trip should take place "once he becomes the nominee after the convention." Sater and Cohen communicated extensively about plans for Cohen's own prospective trip, filings show, but in June 2016, Cohen told Sater he wouldn't go. In court Thursday, Cohen added: "I would like to note that I did not in fact travel there, nor have I ever been to Russia."

After Cohen outlined his lies concerning the extent of his discussion with Trump and his plans for travel to Russia, a special counsel prosecutor, L. Rush Atkinson, told US District Judge Andrew Carter that Cohen had neglected to admit to a third set of falsehoods -- those concerning his direct contact with the Russian government about the Moscow project.

Cohen had had a 20-minute phone call with a representative of the Kremlin in January 2016, according to filings. Atkinson was joined in court Thursday by special counsel prosecutors Jeannie Rhee and Andrew Goldstein.

The charge to which Cohen pleaded Thursday carries a maximum sentence of five years in prison, a maximum fine of $250,000 and supervised release of no more than three years.

Democrats vow to investigate

House Democrats, who will retake the majority early next year, plan to use their new power to investigate the disclosures revealed in the new set of charges against Cohen.

California Rep. Adam Schiff, the incoming House Intelligence Committee chairman, said his panel will try to bring in Cohen and investigate whether there was any money laundering by the Russians through the Trump Organization.

"If Mr. Cohen misled the Congress about the President's business dealings in Russia deep into the campaign, it also means that the President misled the country about his business dealings, and that the Russians were apparently attempting to gain financial leverage over the potential President of the United States," Schiff told CNN's Christiane Amanpour.

Status of Mueller investigation

Trump has criticized the Mueller probe and his own Justice Department almost daily, fired Attorney General Jeff Sessions earlier this month and has hinted at a pardon for his former campaign chairman Paul Manafort.

But Cohen is the latest former Trump associate to say he lied -- apparently to protect the President -- a circle of defense that now seems to be quickly unraveling.

The news comes a few weeks after the President installed a skeptic of the Russia probe, Matt Whitaker, as his acting attorney general. The move was seen by some critics as an attempt to disrupt Mueller and to stop him from making new indictments.

The latest charges also cross what Trump has set as a red line -- an investigation into his family's business empire -- that the President has warned would cause him to fire the special counsel.

This story has been updated.