The justice department has backed special counsel Robert Mueller over a lawsuit filed against him by Paul Manafort, Donald Trump’s former campaign manager, and argued that the case should be dismissed.

Manafort sued Mueller on 3 January, saying his office’s investigation into possible collusion between the Trump campaign and Russia in the 2016 election exceeded its legal authority.

The civil lawsuit, filed in US district court for the District of Columbia, accused the deputy attorney general, Rod Rosenstein, who appointed Mueller, of exceeding his legal authority to “grant Mr Mueller carte blanche to investigate and pursue criminal charges in connection with anything he stumbles across”.

“These claims lack merit,” a justice department filing to the court on Friday said. “The special counsel’s investigation and prosecutions are entirely lawful.”

The department said the case should be dismissed.

Q&A What is the Nunes memo? Show The memo was written by aides to Devin Nunes, chairman of the House intelligence committee and a member of the Trump transition team. The committee is investigating Russian interference in the 2016 election but the inquiry has devolved into a fight about the ​separate FBI investigation​, now​ led by special counsel Robert Mueller​. On Friday, Nunes published the memo after Donald Trump declassified it. The memo revolves around a wiretap on Carter Page, an adviser to the Trump campaign, alleging the FBI omitted key information when it applied for the wiretap. The findings “raise concerns with the legitimacy and legality of certain DoJ and FBI interactions” with the court that approves surveillance requests, the memo says. It also claims “a troubling breakdown of legal processes established to protect the American people from abuses”. The memo criticizes investigators who applied for the wiretap, saying they used material provided by an ex-British agent, Christopher Steele, without sufficiently disclosing their source. The memo says Steele was “desperate that Trump not get elected”. The memo also says texts between an FBI agent and FBI attorney “demonstrated a clear bias against Trump” and says there is “no evidence of any co-operation or conspiracy between Page” and another Trump aide under investigation, George Papadopoulos. The memo casts deputy attorney general Rod Rosenstein in a negative light. ​Rosenstein could fire Mueller. The president, said to dislike Rosenstein, could fire and replace him. The FBI ​argued against the memo’s release. Democrats wrote a rebuttal and sided with the bureau. ​The president reportedly told associates he believes the memo will help discredit the special counsel. Alan Yuhas

Mueller’s office indicted Manafort and his business partner Rick Gates in October on charges including conspiracy to launder money, conspiracy against the United States and failing to register as foreign agents of Ukraine’s former pro-Russian government.

Trump and his Republican allies in Congress on Friday escalated a campaign against US law enforcement agencies over their investigation of the president’s ties to Russia, releasing a disputed memo that the FBI warned was misleading and inaccurate.

Democrats said the four-page memo mischaracterized highly sensitive classified information and was intended to undermine Mueller’s criminal inquiry into the Russia matter launched in May 2017 that grew out of an earlier FBI investigation.

They warned Trump against using it as a pretext to fire Rosenstein or Mueller himself.

In a tweet on Saturday, Trump said the memo “totally vindicates ‘Trump’ in probe. But the Russian Witch Hunt goes on and on.

“Their [sic] was no collusion and there was no Obstruction (the word now used because, after one year of looking endlessly and finding NOTHING, collusion is dead). This is an American disgrace!”