One of the two Trump campaign aides charged in Robert Mueller's Russia probe is stepping up his legal fight to clear his name with friends holding a fundraiser for him next week - in a calculated gesture of Republican defiance at the special counsel.

Rick Gates is having a pre-Christmas fundraiser held to help pay for lawyers to fight federal charges brought against him by Mueller.

The event is being held next Tuesday in Arlington, Virginia, with a goal of raising $250,000 for the 'Defending American Rights Legal Fund' which will benefit the political consultant.

The fund also aims to help other Americans who find themselves facing charges from Mueller's investigation into Russian interference in the 2016 election campaign, according to a GOP source.

The move comes amid mounting questions among Republicans over the integrity of the Mueller probe, after the revelation that one of its most senior FBI agents exchanged anti-Trump texts with his lover, a Department of Justice official.

Fundraiser: Rick Gates remains under house arrest and was last seen on Monday at federal court in Washington. Now friends plan to raise $250,000 towards his legal expenses after he was charged by Special Counsel Robert Mueller

Also accused: Gates was the right-hand man of Paul Manafort and worked for the Trump campaign when Manafort, a veteran lobbyist, was its chair

Gates, 45, is a long-time business associate and protégé of Paul Manafort, the man who ran Trump's presidential campaign for several months in 2016 and is also charged by Mueller.

Manafort, 68, was fired shortly after the Republican National Convention amid questions about his business dealings in Ukraine but Gates stayed on and was more recently working to help close the financial books of Trump's inaugural committee. Gates was in turn removed following questions about his involvement with Manafort.

In October, the two men pleaded not guilty to a 12-count indictment by a federal grand jury, telling a judge that they planned to fight the Justice Department in court.

The business partners face 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 for behavior occurring as far back as 2012. The court documents make no mention of contact with Russians during the presidential campaign.

The government alleges that at least $75 million was moved by Manafort to offshore accounts without declaring the income to the government. From there, Manafort allegedly withdrew $18 million to fund a lavish lifestyle, and Gates is accused of pulling out another $3 million.

Gates is currently under house arrest on a $5 million bond and wearing an ankle bracelet monitor at his home in Richmond, Virginia.

It is not yet clear if he will be able to attend the fundraiser next week but a court decision is due in the coming days, an insider told DailyMail.com.

Earlier this month, Manafort reached an $11.65million bail agreement with the special counsel, allowing him to be released from house arrest and electronic monitoring.

GOP lobbyist Jack Burkman, who is hosting Gates' fundraiser, said he expects a crowd of around 150 DC movers and shakers to attend including members of Congress from both sides of the aisle, fellow lobbyists, lawyers and defense contractors.

Burkman told DailyMail.com: 'I can't get into names but I have a strong sense of support for Rick and Paul throughout Republican Washington, the Trump circles and throughout DC generally. I even know Democratic friends and liberals who feel [Mueller's investigation] is abusive.'

Burkman said he believed that both Gates and Manafort were innocent of the charges but could not get into the details of defense strategy. 'I think Mueller has trumped up a lot of nonsense. He has spent millions of dollars and has to justify his existence so he empaneled the grand jury and now he's looking for any charges he can but he is hurting innocent people.'

He added: 'I think the facts will speak for themselves. It's very simple – I don't think Rick Gates has done what Mueller is alleging. I think Mueller is desperate – and his strategy is to charge people, force them to spend money they don't have and try to pressure them into some kind of plea. He did that with Michael Flynn by attacking Flynn's son. He terrorized Flynn's family.

'I've had deep feelings against special counsel Bob Mueller for some time because I believe he has unfairly charged both Rick Gates and Paul Manafort. I think he's abused his authority in every way a special counsel can. It started off with an investigation into the president and now he's going off into very tangential areas, looking for the smallest of things.

'I think he's just abusing these two poor men and we want to help. Paul Manafort has a lot of wealth, he doesn't need any fundraiser but Rick Gates has four kids, the poor man is suffering. He is a wonderful man and he is being abused by this terrible devil Bob Mueller.'

The charges against Gates and Manafort stem from Mueller's investigation into the conclusions of U.S. intelligence agencies that Russia undertook a campaign of hacking and misinformation to tilt the election in Trump's favor and potential collusion by Trump associates.

Questions: The revelation that Lisa Burrow and Peter Strzok exchanged thousands of texts riddled with anti-Trump rhetoric has rattled the Mueller probe. He was seconded to it after playing a key role in the FBI investigation which cleared Hillary Clinton

No collusion: Trump repeated his denial Friday that he had in any way colluded with Russia. He also highlighted questions over the FBI's integrity, saying its reputation was in 'tatters'

The possible sentences, if found guilty, would be stiff: up to 80 years in prison for Manafort and 70 years for Gates, plus millions of dollars in fines. More typical federal sentencing for the crimes charged would be 10 to 12.5 years for Gates and 12.5 to 15 for Manafort. Trump insisted via Twitter shortly after the indictments that there was 'NO COLLUSION' and Manafort's lawyer later said that the president 'was correct'.

'There is no evidence that Mr Manafort or the Trump campaign colluded with the Russian government,' Kevin Downing said.

The move to throw Manafort overboard came after reports that Manafort pocketed at least $12 million in undisclosed payments from Viktor Yanukovych, the pro-Russia former Ukrainian president.

Manafort and his firm, which Gates helped run, spent years as Yanukovych's political consultants.

The Justice Department's indictment charges that Manafort and Gates 'generated tens of millions of dollars in income as a result of their Ukraine work'.

'In order to hide Ukraine payments from United States authorities, from approximately 2006 through at least 2016, Manafort and Gates laundered the money through scores of United States and foreign corporations, partnerships and bank accounts,' it continues.

Mueller's probe focused on wire transfers Manafort made from Ukraine to private accounts and whether he paid taxes on that income.

Kevin Downing, Manafort's lawyer, was formerly a prosecutor in the Justice Department's criminal tax division.

In July the FBI in July executed a surprise search warrant with guns drawn at Manafort's home in Alexandria, Virginia, seizing financial and tax documents.

The Russia investigation has cast a shadow over President Trump's nine-month-old presidency and widened the partisan rift between Republicans and Democrats.

U.S. intelligence agencies concluded in January that Russia interfered in the election to try to help Trump defeat Democrat Hillary Clinton by hacking and releasing embarrassing emails and disseminating propaganda via social media to discredit her.

Mueller is also investigating whether Trump campaign officials colluded with those Russian efforts.

Trump has denied the allegations of collusion with the Russians and called the probe 'a witch hunt.' The Kremlin also has denied the allegations.