WASHINGTON (BLOOMBERG) - President Donald Trump and his lawyer fired what may be the first shots in a showdown with Special Counsel Robert Mueller over the future of his investigation into Russian election meddling and Mr Trump's campaign.

The firing of Mr Andrew McCabe, until recently the FBI's deputy director, prompted Mr Trump's lawyer, Mr John Dowd, to call for Mr Mueller to shut down his collusion probe.

Mr Trump, meanwhile, unleashed a pair of tweets attacking the Federal Bureau of Investigation and Mr James Comey, the FBI director he fired in May.

The comments were the latest sign that Mr Trump has lost patience with the months-long investigation that's cast a dark shadow over his presidency.

Mr Trump's lawyers, who have been negotiating terms for Mr Mueller to interview the president, had assured their client for most of last year that the investigation would wrap up by the end of 2017, said a person familiar with the matter.

Mr Trump was talked out of firing Mr Mueller back in June, but there are strong signals that the special counsel and his team of 17 prosecutors have at least several months more work ahead of them.

"The Mueller probe should never have been started in that there was no collusion and there was no crime," Mr Trump said Saturday on Twitter. "It was based on fraudulent activities and a Fake Dossier paid for by Crooked Hillary and the DNC, and improperly used in FISA COURT for surveillance of my campaign. WITCH HUNT!"

Any move to fire Mr Mueller is expected to ignite a political firestorm in Washington. Democrats have warned of a constitutional crisis, and even most Senate Republicans have cautioned Mr Trump against doing anything to curtail the special counsel's investigation.

Republicans were noticeably silent, though, in the immediate aftermath of the new calls from Mr Trump and his lawyers to end the probe. It is unclear whether they would take steps to rein the president in if he took drastic action.

Mr McCabe's firing also adds fresh fuel to Mr Mueller's probe.

Mr Michael Bromwich, a former Justice Department attorney now serving as one of Mr McCabe's lawyers, said that the veteran FBI agent was fired after the disclosure that he was a cooperating witness against Mr Trump.

Mr McCabe documented his interactions with Mr Trump in a series of memos, according to a person familiar with the matter, and those memos could play into Mr Mueller's investigation. The memos have been provided to the special counsel's office, according to Associated Press.

Now that Mr McCabe has lost his job and possibly a substantial portion of the pension accrued in more than two decades with the FBI, he has little reason not to speak out - starting with the lengthy statement on Friday night where he noted that he could "corroborate former director Comey's accounts of his discussions with the president".

Mr McCabe is "a loose cannon right now. Talk about a guy who has nothing to lose - literally, nothing to lose", said Mr Jeffrey Cramer, a former federal prosecutor who's now managing director of the international investigation firm Berkeley Research Group.

"If he was holding anything back out of loyalty to the FBI or a sense of duty, well, that just walked out the door. If he has any information he hasn't revealed out of a sense of loyalty, that might be told now," Mr Cramer said.

Mr Comey, meanwhile, is about to embark on a high-profile publicity tour and series of television interviews to promote his memoir, A Higher Loyalty: Truth, Lies and Leadership, which will be released on April 17.

"Mr President, the American people will hear my story very soon," Mr Comey said on Saturday in a tweet. "And they can judge for themselves who is honourable and who is not."

The White House on Saturday did nothing to clear up the confusion about Mr Trump's stance towards Mr Mueller's probe.

"I pray that Acting Attorney General Rosenstein will follow the brilliant and courageous example of the FBI Office of Professional Responsibility and Attorney General Jeff Sessions and bring an end to alleged Russia Collusion investigation manufactured by McCabe's boss James Comey based upon a fraudulent and corrupt Dossier," Mr Dowd said in an e-mailed statement early in the day.

He quickly clarified that he was speaking for himself, not for the president.

Even so, Mr Trump cheered the firing of Mr McCabe, calling the FBI corrupt, and claiming the House Intelligence Committee found "there was no collusion between Russia and the Trump campaign".

The full committee released no such finding, although Republicans on the panel said they found no evidence of collusion.

The White House did not respond to multiple requests for comment. White House lawyers have said in the past they plan to cooperate fully with Mr Mueller's probe, but Mr Trump's other personal lawyer, Mr Jay Sekulow, has used his daily radio show to chip away at Mr Mueller's investigation for months.

The show, which Mr Sekulow has broadcast for more than two decades to an audience of more than 1.5 million daily listeners, has become a key venue for challenging the basis for Mr Mueller's investigation. Mr Sekulow has regularly attacked the credibility of Mr McCabe and other FBI officials looking into Russian election meddling, as well as the use of a surveillance warrant against a Trump campaign adviser.

Democrats said they were alarmed by the latest attacks on the investigation.

"Any attempt by the president to obstruct or remove the special counsel would create a constitutional crisis and represent an attack on the core American principle that nobody, including the president of the United States, is above the law," warned Democratic Senator Chris Coons.

The president cannot fire Mr Mueller directly, since he answers to the Justice Department official who appointed him, Deputy Attorney General Rod Rosenstein. Usually a special counsel would report to the attorney general, but Mr Sessions recused himself from the inquiry because he advised and supported Mr Trump's 2016 campaign. But Mr Trump still has options.

A series of potentially pivotal events are on the horizon for the investigation. Beyond a possible interview of the president, Mr Mueller still has not had wide-ranging interviews with members of Mr Trump's family who were witnesses to some of the issues he is investigating.

While Mr Mueller is believed to be nearing the end of his investigation into whether Mr Trump obstructed justice, he may hold off on releasing any conclusion until other parts of his probe are settled, current and former officials said.

Mr Mueller also appears to be looking into Mr Trump's business, including asking witnesses about a proposed Trump Tower Moscow. The New York Times reported Mr Mueller issued a subpoena for documents to the Trump Organisation several weeks ago.

Mr Dowd had previously said it would be outside Mr Mueller's mandate to probe into Mr Trump's business dealings. He declined to comment on the subpoena.

Given Mr McCabe's role as an important witness with details about the firing of Mr Comey, his termination could add to - but not derail - an obstruction of justice case, said Mr William Yeomans, a 26-year Justice Department veteran who has served as an acting assistant attorney general.

"If anybody had any doubts about the integrity of this process, they were put to rest by the president's tweet, which basically announced he had forced this, and it was a good thing that this long-serving FBI employee who had done some wonderful things during his career was going to be forced out two days before he qualified for his retirement," Mr Yeomans said in an interview.

Mr Sessions fired Mr McCabe ahead of his planned retirement on Sunday at age 50, a move that Mr Trump celebrated on Twitter as a "a great day for Democracy".

Mr Sessions said he was responding to a report by the Justice Department's inspector general and finding by the FBI's Office of Professional Responsibility. Those offices found that Mr McCabe had not been fully forthcoming with investigators in discussing his contacts with a reporter, according to a person familiar with the matter.

Mr McCabe, who joined the FBI in 1996, became a Republican target partly because he helped oversee the investigation into Democrat Hillary Clinton's e-mail practices in 2016, even though his wife had accepted donations from Democratic political organisations during a losing campaign for the Virginia state Senate the previous year.

"How many hundreds of thousands of dollars was given to wife's campaign by Crooked H friend, Terry M, who was also under investigation?" Mr Trump said in an earlier tweet. "How many lies? How many leaks? Comey knew it all, and much more!"