The special counsel Robert Mueller has thrown a wrench into the core of former Trump campaign chairman Paul Manafort's defense strategy in the Russia investigation.

Manafort's lawyers have argued that Mueller overstepped the bounds of his mandate by charging him with crimes unrelated to Russian collusion, and that Mueller's mandate itself is too broad.

But Mueller's office revealed a previously undisclosed memo on Monday, which specifically gave him the authority to investigate matters related to Manafort's lobbying work in Ukraine.

Mueller also argued that Manafort does not have the right to determine the validity or scope of his appointment.

The special counsel Robert Mueller's office revealed late Monday night that it had received a private memo from deputy attorney general Rod Rosenstein authorizing him to investigate Paul Manafort's lobbying work in Ukraine before he joined President Donald Trump's campaign as chairman in 2016.

The existence and content of Rosenstein's memo, which was dated August 2, 2017, has not been previously reported, and its revelation appears to significantly hamper Manafort's defense strategy in the Russia investigation.

Manafort's defense is predicated on the claim that Mueller overstepped his mandate by charging Manafort with crimes not specifically related to the collusion inquiry. His lawyer has also argued that Mueller's mandate itself is "tantamount to a blank check."

Rosenstein's memo was one part of a larger court filing Mueller's office submitted Monday as a defense of its investigative strategy on Manafort so far. The memo's purpose was to outline a more specific description of Mueller's authority following his appointment as special counsel last year. While much of the document is redacted to protect the identities of specific individuals and investigative paths Mueller may be following, the portion related to Manafort has been unredacted.

According to the memo, Mueller is authorized to investigate two broad threads related to the former Trump campaign chairman:

Whether he colluded with Russian government officials as Russia was trying to meddle in the 2016 US election;

Whether he committed any crimes "arising out of payments he received from the Ukrainian government before and during the tenure of President Viktor Yanukovych."

Manafort was unaware of the memo's existence before it was publicly revealed Monday night, and the second stipulation throws a wrench into his claim that Mueller overstepped his mandate by charging him with crimes unrelated to Russian collusion.

Mueller's office has so far charged Manafort with dozens of counts related to his work as a top consultant to Viktor Yanukovych, the former president of Ukraine and a pro-Russian strongman.

Manafort has been charged with financial crimes like tax and bank fraud, money laundering, and conspiracy against the US. He has also been charged with making false statements to FBI agents about his lobbying work and financial background.

He's pleaded not guilty to all of the charges against him.

Mueller says Manafort does not have the right to decide what the special counsel can do

Paul Manafort's lawyers have argued Mueller's investigation has gone off the rails. Thomson Reuters

Manafort's attorney filed a motion last month asking a federal judge in Virginia to dismiss the most recent charges against him, arguing that Mueller was casting too wide a net while investigating Manafort.

The special counsel's office also addressed that assertion in its filing Monday night, arguing that Manafort's objections to the scope of Mueller's mandate were "unsound."

It added that the DOJ's regulations regarding the appointment of a special counsel give Mueller "limited flexibility" while authorizing Rosenstein to amend the scope of his mandate where "necessary in order to fully investigate and resolve the matters assigned."

Mueller's office also questioned Manafort's right to argue the validity of the special counsel's mandate at all, saying Manafort had "no basis" to use that reasoning to call for the case against him to be dismissed.

Specifically, Mueller's office said the DOJ regulations governing the appointment of a special counsel are meant to provide a framework for the department's internal structure. The regulations "unequivocally state" that they are not meant to "create any [enforceable] rights" in a criminal proceeding, the special counsel continued.

When Rosenstein tapped Mueller for the position last year, he gave Mueller the authority not only to investigate "any links and/or coordination between the Russian government and individuals associated" with Trump's campaign, but also to examine "any matters that arose or may arise directly from the investigation."

Rosenstein also gave Mueller the power to investigate "any other matters within the scope" of his mandate, including perjury, obstruction of justice, destruction of evidence, and intimidation of witnesses.

The deputy attorney general testified to the House Judiciary Committee last year that he did not see good cause to fire Mueller.

"There are a lot of media stories speculating about what the special counsel may or may not be doing," Rosenstein told GOP Rep. Lamar Smith, who expressed concerns that Mueller was "casting too wide of a net" in the investigation.

"I know what [Mueller] is doing," Rosenstein said. "I'm appropriately exercising my oversight responsibilities. So I can assure you that the special counsel is conducting himself consistently with our understanding about the scope of his investigation."

Read the full filing below: