WASHINGTON — President Donald Trump has taken aim at many targets over time and often says he’s just counterpunching his critics. Yet there’s one perceived foil he goes after time and again without provocation or much threat of a backlash — his predecessor, Barack Obama.

“Great timeline on all of the failures the Obama administration had,” he tweeted to congratulate his favorite Fox News show recently. “Obama did nothing about Russia!” he tweeted days later. Repeatedly he has said Obama is the one who should be investigated, not him, because Russia’s interference in the 2016 campaign happened on Obama’s watch.

Trump has slammed Obama about health care, the Iran nuclear deal, the economy, gun and immigration policy and even (falsely) for the relocation of the U.S. Embassy in London — and that’s just in the 80-plus tweets he’s fired off against his predecessor, not counting his public remarks. In his most memorable attack of all, a year ago Trump charged, without evidence, that Obama ordered the “wires tapped” in Trump Tower, adding, “This is McCarthyism!”

Obama is no shrinking violet, and relished the occasional sharp retort. “You’re likable enough, Hillary,” was an early, memorable one. But in the post-presidency, Obama mostly is mute. When he does speak out, he never explicitly mentions the president.

The same goes for the other four living ex-presidents back to Jimmy Carter. Obama, as Trump’s immediate predecessor and the one especially reviled among the president’s white working-class base, is the more frequent target. Yet the other three presidents of the past quarter-century — both George Bushes, father and son, and Bill Clinton — have taken frequent hits, often collectively, as Trump indicts them all for some perceived failure.

Turning the other cheek is a new phenomenon for an elite group known as “The President’s Club” for their experiential bond that transcends partisanship. Until now, cheek-turning wasn’t necessary: By long-standing tradition, past presidents didn’t publicly attack their predecessors, or vice versa, once the campaigns ended.

George W. Bush kept his thoughts to himself during the Obama years, just as his predecessor, Clinton, did for Bush and as President George H.W. Bush did for Clinton, though Clinton had ousted him from office. Like them, Obama heeds the old customs even as the newest member of the club — Trump — flouts them.

“Obama certainly had critical things to say about Trump when he was running, and both of the Bushes said they weren’t going to vote for Trump,” said James Thurber, a presidential scholar.

“But we haven’t heard from them since he became president, and the reasoning is that they have respect for the office of the presidency,” Thurber said. “We have one president at a time and they respect that.”

By contrast, Trump recently showed again that he doesn’t return the respect, tweeting that George W. Bush didn’t have the “smarts” to get along with Russia, while Clinton and Obama “didn’t have the energy or chemistry.”

After last summer’s solar eclipse, Trump singled out Obama, retweeting a series of photos of Obama and himself in which his face moved to cover Obama’s. The caption: “THE BEST ECLIPSE EVER!”

Most presidents and ex-presidents have criticized each other gently, if at all, said Joanne Freeman, an early American history scholar at Yale University.

“That isn’t to say that presidents haven’t ever critiqued each other’s policies. They occasionally have,” she said. “But they usually focus on policies, rather than tossing around insults and accusations.”

The tradition of new presidents not assailing their predecessors dates to the country’s start. As the second man to hold the office, John Adams was so concerned about honoring the service of the first, George Washington, that he didn’t even replace Washington’s Cabinet appointees.

The third president, Thomas Jefferson, assumed the office after a particularly nasty campaign and yet, despite his deep disapproval of the Federalist policies of the two preceding administrations, he did not attack Adams’ record.

Presidents and ex-presidents have criticized each other before, but not with Trump’s regularity. For example, Carter attacked George W. Bush, especially after the invasion of Iraq, calling his foreign policy “the worst in history” and his faith-based social program “quite disturbing.”

Bush, who left office highly unpopular, amid two wars and the worst recession and financial crisis since the Great Depression, kept quiet when Obama occasionally complained of the “big mess” he’d inherited. Yet Obama avoided using Bush’s name.

As Obama was preparing to leave office, aides said, he talked with them about how he would carry himself through what promised to be a brash and bombastic Trump presidency.