President Obama's final year in office — which was expected to be a time when he would give symbolic speeches and try to cut a few deals with Republicans — is turning into an all-out campaign to stop Donald Trump from succeeding him.
In recent public remarks, including his endorsement of Hillary Clinton even before the Democratic primaries were finished, Obama has inserted himself more directly into the campaign than previous second-term presidents.
He is effectively turning the election into "Obama-Clinton v. Trump," instead of a traditional contest between the two people with their names on the ballot.
"This is the biggest intervention that I can remember for some time now," said Julian Zelizer, a political historian at Princeton University, comparing Obama to past second-term presidents. "Not only is he really coming out for the Democratic candidate, but acting in extremely aggressive fashion against the Republican nominee."
Obama's behavior appears to be motivated in part by Trump's unorthodox campaign style and platform, including the proposal to temporarily ban Muslims from entering the United States that has particularly alarmed the president.
"Islam has always been part of America," Obama said in a visit to a mosque in Baltimore in February.
"The president will give this campaign all that he has and more — for Clinton, for his own legacy, and for the vision of America that he's asked us to believe in since the night he stepped onto the national stage in Boston and delivered his 2004 convention speech, a hopeful, bighearted vision that is the antithesis of everything that Trump represents," former Obama speechwriter Jon Favreau wrote in a recent piece for The Ringer, a sports and pop culture website.
Last Tuesday, Obama dismissed Trump's comments on the Orlando shooting and the fight against ISIS in blunt, fiery terms that usually don't come from an outgoing president in describing his potential successor.
"We're starting to see where this kind of rhetoric and loose talk and sloppiness about who exactly we're fighting, where this can lead us," the president said last week after he met with his national security advisers. "We now have proposals from the presumptive Republican nominee for president of the United States to bar all Muslims from immigrating to America. We hear language that singles out immigrants and suggests that entire religious communities are complicit in violence. Where does this stop? The Orlando killer, one of the San Bernardino killers, the Fort Hood killer — they were all U.S. citizens."
The president is expected to hold a campaign event soon with Clinton, breaking with the tradition of past second-term presidents, who often only get heavily involved on the trail after the party nominating conventions. A joint Clinton-Obama event that was scheduled for Wednesday in Wisconsin was postponed after the Orlando shooting.
And Obama, starting with his State of the Union address in January, has spent much of this year criticizing Trump's ideas and imploring the media to cover him more critically.
"We are in serious times and this is a really serious job. This is not entertainment. This is not a reality show. This is a contest for the presidency of the United States," Obama said last month. "And what that means is that every candidate, every nominee needs to be subject to exacting standards and genuine scrutiny."
He added, "One thing that I'm going to really be looking for over the next six months is that the American people are effectively informed about where candidates stand on the issues, what they believe, making sure that their numbers add up, making sure that their policies have been vetted and that candidates are held to what they've said in the past."
In most campaigns nearing the end of a two-term presidency, it's the challenger who attacks the incumbent.
In 2000, George W. Bush suggested he would return "honor and dignity" to the White House, a reference to the sex scandals under Bill Clinton.
In 2008, Obama constantly blasted Bush for the Iraq War.
Neither president engaged with the presidential hopefuls in a back-and-forth over the jabs.
But the Obama/Trump dynamic is different and goes way back.
Trump was the informal leader of the efforts a few years ago to suggest falsely that Obama was not born in the United States. And in his 2016 campaign, the real estate mogul has repeatedly repudiated the president's policies, particularly Obama's refusal to use the term "radical Islam" in describing ISIS.
In the past, sitting presidents were also largely deferential as their successors wanted to remain the central figures in the campaign.
"Reagan was lukewarm about Bush, Gore kept Clinton at arms-length because of the scandals, and McCain didn't really want to associate himself with such an unpopular president," Zelizer said.
In contrast, Clinton aides view Obama, with his relatively high approval rating overall and near universal backing from Democrats, as an asset.
"It was powerful today to have both @POTUS & @HillaryClinton standing up at the same time against Trump and in defense of our values #InSync," Clinton spokesman Brian Fallon wrote in a Twitter message on Tuesday.
To be sure, the rise of Trump is not just causing Obama to take unusual steps. Many members of Trump's own party won't endorse him, which is almost unprecedented for a presidential candidate.
And Obama has a more selfish reason to stop Trump: Protecting his policies, such as Obamacare, which the real estate mogul has pledged to repeal.
"I think that because Obama has really seen how aggressive congressional conservatives have and will be with his agenda — fearing that much of it might be dismantled if Republicans are in control — he really sees the stakes in who controls the White House," Zelizer said.