It must be one of the Western world’s most thankless tasks. Being Donald Trump’s lawyer involves not simply weathering your client’s infamous tirades but seeing your careful advice blown away in a 140-character Twitter storm.

But Marc Kasowitz, who represents Mr Trump’s personal interests in the snowballing investigation into alleged collusion between his campaign and the Kremlin during last year’s election, is hardly a newcomer to the president’s court.

Mr Kasowitz, who proudly boasts of his reputation as an “uberlitigator” and the “toughest of the tough guys”, has worked for Mr Trump for 15 years. He is, one client told Bloomberg, “the type of attorney people turn to when seeking to intimidate the other side”.  

Not all of Mr Trump’s foes have succumbed, though. Last year, at a cost of an estimated $25m, Mr Kasowitz helped the then president-elect settle a fraud case over Trump University.

During the election campaign, the New York Times refused to give way when Mr Kasowitz threatened to sue the paper for “per se libel” for reporting the case of two women who accused Mr Trump of groping them. Reminding Mr Kasowitz that libel laws are designed to protect a person’s reputation, the Times’ lawyers shot back: “Nothing in our article has had the slightest effect on the reputation that Mr Trump, through his own words and actions, has already created for himself.”

Some of the cases the president has demanded Mr Kasowitz file have been an even taller order: in 2006, Mr Trump attempted to sue an author for $5bn for allegedly under-estimating his wealth. The suit was later thrown out by a judge.

The closeness between the president and Mr Kasowitz is underlined by the fact that, even in the White House, Mr Trump has often turned to his law firm. David Friedman, a former partner, was tapped to become US Ambassador to Israel, while former Senator Joe Lieberman, a counsel to the firm, was at one time under consideration for the post of FBI director.  

The Russia investigation, however, appears to be putting the partnership between Mr Trump and Mr Kasowitz under increasing strain. After the revelations about Donald Trump Jnr’s secret meeting with a Kremlin-linked lawyer who allegedly offered damaging information about Hillary Clinton, the president reportedly turned on Mr Kasowitz. US media outlets last week suggested that Mr Trump has grown disillusioned with his lawyer’s strategy.

Mr Kasowitz, in turn, is said to be deeply frustrated by the president’s unwillingness to follow his advice and stop Tweeting about the case and discussing it with others – like Mr Trump’s son-in-law, Jared Kushner – who are also under investigation. Mr Kasowitz is also believed to be concerned that Mr Kushner is more focused on protecting himself than the president. Other reports suggest that Mr Kasowitz has been shocked by West Wing infighting and irritated about often not finding out about major developments in the investigation until the last minute.

Last week, Mr Kasowitz’s apparent frustration appeared to boil over dramatically. It was triggered by a news report which suggested that, despite the highly sensitive nature of the Russia probe, Mr Kasowitz had not applied for security clearance, allegedly due to a struggle with alcohol addiction, which the lawyer denies. Mr Kasowitz’s spokesman told investigative website ProPublica he did not need clearance. “No one has suggested he requires a security clearance, there has been no need for a security clearance, and we do not anticipate a need for a security clearance,” the spokesman said. “If and when a security clearance is needed, Mr. Kasowitz will apply for one with the other members of the legal team.”

When a member of the public emailed the president’s lawyer politely suggesting he might not be best suited to represent Mr Trump, he received a volley of abusive emails in reply. An initial email saying “f--- you” was followed by a series of more expansive responses. “I’m on you now. You are f---ing with me now … Watch your back, bitch,” wrote Mr Kasowitz in one, while another threatened: “I already know where you live, I’m on you … You will see me. I promise. Bro.”

Whatever their current frustrations, Mr Kasowitz’s emails – so redolent of Mr Trump himself – will no doubt rekindle the president’s affection and regard for his lawyer.