Washington (CNN) When President Donald Trump on Monday suggested his predecessor had failed to contact the families of fallen American troops , it was less a bombshell and more a twist in what has become one of his presidency's ongoing subplots.

As Trump enters his tenth month in office, Barack Obama remains his most dependable antihero, with the slights, barbs and outright lies he's uttered about his predecessor mounting.

Since January 20, Trump has accused Obama of: s urveilling him at Trump Tower ; ignoring evidence of Russian election meddling; allowing a nuclear threat to ferment in North Korea ; approving embarrassing international agreements in the Paris climate accord and the Iran nuclear deal; masterminding a failed health care law; over-regulating the energy sector and killing coal jobs; micromanaging the battle against ISIS; and standing by as Syria's dictator used chemical weapons on civilians.

Even before last year's presidential campaign -- which featured heated arguments from both Trump and Obama about the other's record -- there existed deep animosity between the two men. Trump was a chief propagator of the false birther myth, which claimed Obama wasn't born in the United States. Obama stoked Trump's anger by ruthlessly roasting him at a 2011 black-tie dinner they both attended.

Men at odds

Now, as the current and most recent presidents, the men remain at odds. Trump has aggressively worked to reverse key elements of Obama's legacy, including his health care law and his negotiated agreements on climate change and Iran's nuclear program . And he rarely allows an opportunity to pass without launching criticism of his predecessor.

"If you look at President Obama and other presidents, most of them didn't make calls," Trump said Monday when asked about his delayed response to the deaths of four US soldiers in Niger . "A lot of them didn't make calls. I like to call when it's appropriate."

The remark drew swift and bitter reprisals from Obama's former aides, who pointed to the former president's meetings, calls and letters sent to the families of fallen US service people as evidence of a false claim. But the backlash only seemed to embolden Trump further. He suggested during a radio interview a day later that Obama hadn't phoned Gen. John Kelly, the current White House chief of staff, when his son was killed in Afghanistan in 2010 (Obama did meet with Kelly and his wife during a 2011 breakfast at the White House).

The episode featured all the usual set pieces for Trump's now-frequent censures of Obama: deflecting blame from questions about his own actions, boasts about the superiority of his response and a misleading or outright false claim about the man who held the job before him.

"It bothers me that a president of the United States, instead of accepting responsibility for what he does and what his administration does, constantly looks for other scapegoats, whether it's Congress, whether it's past presidents, whether it's somebody else," said Leon Panetta, who served as defense secretary under Obama. "He is never responsible for anything that goes wrong. And the reality is the American people understand that presidents make mistakes."

White House officials downplayed Trump's frequent criticism of Obama, saying he was elected on promises to undo much of his agenda. People who know Trump say he's long sought out adversaries as a way to help define and articulate his own vision -- in his jobs as a real estate developer and reality television personality, and now as President. Over the last several months, Trump hasn't reserved his ire just for Obama; he's lambasted everyone from Hillary Clinton to the NFL to sitting Republican senators.

'Temporary occupants'

Obama, meanwhile, has issued written statements at various moments decrying steps Trump has taken to dismantle his legacy, including pulling out of the Paris climate deal or ending the DACA program that shields some young immigrants from deportation. But in his public remarks he's avoided direct criticism of Trump, never naming him and referring in only veiled ways to his actions.

When he returns to political campaigning this week for Democratic gubernatorial candidates in New Jersey and Virginia, Obama is expected to similarly avoid directly hitting Trump, according to one of his advisers, who said Obama was unlikely to campaign at the same pace Trump has so far this year.

Trump and Obama met twice after last year's election: once in the days following Trump's victory, and once on Inauguration Day. Those meetings were described as largely cordial, according to people familiar with them, with extended discussions of the issues Trump would face and the pitfalls he could avoid.

That was the last Trump would hear from Obama. Dormant, for now, is the long-running custom of informal consultation between the occupant of the White House and its previous inhabitants. Trump hasn't contacted Jimmy Carter, George H.W. Bush, Bill Clinton, George W. Bush or Obama to discuss the myriad world crises he faces, despite their experience in comparable circumstances.

When the living former presidents banded together to create a charity appeal for victims of the recent spate of hurricanes , Trump wasn't asked to participate (he later tweeted his support). It was a different approach than similar efforts after disasters in Southeast Asia and Haiti, when ex-presidents were invited to the White House to launch their relief efforts.

After Trump's remarks this week about contacting families of killed US troops, aides to former presidents avoided public pushback. But privately, many were astounded and appalled that Trump would launch such an easily refuted claim.

The chasm between Trump and his predecessors is another data point in an untraditional presidency, one that has discarded many of the unwritten rules and customs that used to dictate how presidents behave.

"His instinct isn't aligned with what most past presidents who have served in this role, and have been commander in chief, have done," said Jennifer Psaki, a former communications director for Obama. "Why does he do it? I don't know. It may just be part of the fabric of who Donald Trump is, which is disappointing when that's the person sitting in the Oval Office."