President Donald Trump, who has relished standing apart from Washington's political establishment, found himself more isolated than ever Saturday, airing his latest grievances and retreating to his private golf course in Virginia as his peers gathered to pay homage to the late Sen. John McCain.

Trump had not been invited to the memorial service for McCain at Washington National Cathedral, where his two recent predecessors, Barack Obama and George W. Bush, delivered eulogies.

The 45th president, who clashed with McCain, a fellow Republican, did not mention him in a series of morning missives on Twitter. Instead, Trump issued tweets threatening to terminate the North American Free Trade Agreement and promoting a false conspiracy theory alleging government misconduct in its surveillance of one of his former campaign aides.

"I love Canada, but they've taken advantage of our Country for many years!" Trump wrote in one tweet, as his motorcade made its way to the Trump National Golf Club in Sterling, Virginia. The president, spotted briefing by reporters as he left the White House, was dressed in a white, short-sleeve shirt and white MAGA hat.

"If we don't make a fair deal for the U.S. after decades of abuse, Canada will be out," Trump wrote. "Congress should not interfere w/ these negotiations or I will simply terminate NAFTA entirely & we will be far better off."

The sharp dichotomy of Trump's pugilistic posts and the dignified memorial service, broadcast live by cable news stations and online, underscored the president's unwillingness to embrace the traditional duties of office and his scorn of Washington's protocols and conventions, which he has delighted in undermining.

Trump has chafed at the attention showered on McCain, who angered him last year by casting a crucial vote against a repeal of the Obama-era Affordable Care Act. The president rejected an initial draft from aides of an official White House statement that called McCain a "hero," and he begrudgingly ordered the American flag atop the White House re-lowered to half-staff, after it had been raised to full-staff following the minimum time required by law in recognition of the death of a member of Congress.

In another pair of tweets Saturday to his 54 million followers, Trump promoted an article from CNSNews.com, a conservative website, that suggested a recent report from Judicial Watch, a conservative government watchdog group, contained a bombshell revelation: A U.S. surveillance court failed to hold any hearings when granting and renewing warrants to monitor the movements and communications of ex-Trump campaign adviser Carter Page in 2016 and 2017.

Trump quoted Judicial Watch President Tom Fitton alleging "corruption" within the FBI and Department of Justice and the leadership of those agencies "being out to lunch" in seeking accountability.

In fact, the story was baseless, given that the Foreign Intelligence Surveillance Court, which handles warrants for surveillance of suspected spies and terrorists, normally does not hold hearings.

The 11 judges, all appointed by the Supreme Court's chief justice, take rotating turns as the "duty judge" for several weeks at a time, and review written applications from the Justice Department and rarely hold hearings to consider them. The court's proceedings are classified and ex-parte, so no lawyers or witnesses for the suspects are aware of the application for a warrant.

The Washington Post's Carol D. Leonnig contributed to this report.