To back his assertion that President Barack Obama had coddled the world’s top sponsor of terrorists, Rep. Paul A. Gosar (R-Ariz.), cited an unusual source: a clumsily altered image of a nonexistent handshake between Obama and the Iranian president. The doctored photo, once used in TV ads supporting Republican Sen. Ron Johnson of Wisconsin, had been repeatedly debunked since it first surfaced on an Egyptian Islamist political website in 2013.

But when critics last week chided Gosar for showing hundreds of thousands of people a faked image of an imaginary event, the fifth-term congressman said they, the “dim witted” ones, were in the wrong. “No one said this wasn’t photoshopped,” he declared. “The point remains … The world is better without Obama as president."

For ginning up political resentment and accentuating your rivals’ flaws, nothing quite compares to a doctored image. It can help anyone turn a political opponent into a caricature — inventing gaffes, undercutting wins and erasing nuance — leaving only the emotion behind.

Sharing doctored images of an electoral rival is a timeworn strategy of modern politics: In campaign mailers and TV ads, shadowy lighting, sinister music and unflattering facial expressions are so expected as to be cliche. But those tactics are increasingly playing out on the Internet, the most powerful visual medium in history, where they don’t require a campaign’s backing or resources to get attention.

Trolls in the last year have superimposed the late sex offender Jeffrey Epstein’s head onto the body of Democratic Rep. Adam B. Schiff’s father from a family Thanksgiving photo; doctored a Black Lives Matter protest photo with a sign supporting Sen. Elizabeth Warren (D-Mass.) to suggest her campaign had attempted the misrepresentation; and changed a photograph to make it appear that Sen. Bernie Sanders (I-Vt.) was pointing at a supporter’s shirt that read, “America deserved 9/11.”

On Monday, Trump, who has more than 70 million Twitter followers, retweeted a fake image of House Speaker Nancy Pelosi (D-Calif.) and Senate Minority Leader Charles E. Schumer (D-N.Y.) cartoonishly altered to show them in a turban and hijab. The tweet, which falsely claims that the two most powerful Democrats in Congress have “come to the Ayatollah’s rescue,” has been retweeted more than 17,000 times.

White House press secretary Stephanie Grisham told Fox News that “the president was making the point that the Democrats seem to hate him so much that they’re willing to be on the side of the leadership of countries that want to kill Americans.” Jasmine El-Gamal, a former Middle East adviser at the Pentagon, called Trump’s tweet “deeply damaging,” adding, “This President thinks that a good way to insult someone is by painting them as Muslim.”

Sharing altered images and video, long a prized weapon of online trolls and meme-makers, is increasingly being seen as fair game for political leaders, reducing reality into a viral meme and flattening complex personalities into characters evoking only pity, loathing or disgust. The posts apply the hallmarks of political cartoons and parodies to real life, fudging the facts to represent what their distributors argue is a deeper truth: the indecency, inadequacy or idiocy of their enemies.

“They’re not necessarily trying to persuade someone to come to their side,” said Darren Linvill, a professor at Clemson University who researches social media disinformation. “They’re trying to reinforce existing beliefs and get people more entrenched in those beliefs. The more entrenched we are, the less possible it is to agree with the other side."

Misleading fakes have been used against a bipartisan cast of political leaders, including Trump, but many of the most prominent cases have targeted top Democrats. All of the Democratic front-runners for the presidential nomination, as well as the leading Democrats in Congress, have been targeted by deceptively altered images or videos within the last year.

Trump is among their top distributors, routinely sharing crude memes, doctored images and satirical fakes targeting his political opponents. One video, in which former vice president Joe Biden appears to fondle himself, has been retweeted roughly 70,000 times.

Researchers think Trump’s willingness to share altered images may be one reason for the increase: Some of the political meme-makers he’s spotlighted have capitalized on the attention by soliciting donations or charging for premium subscriptions. A study by Princeton and New York University researchers last year, based on surveys and social media data from 3,500 respondents, also found that conservatives and people 65 or older were more likely than other adults to share links to fake-news sites in 2016.

For the images’ creators — and many in their audience — the falseness barely matters. The feelings they evoke are the point, Linvill said, and people who believe the message already aren’t looking for a counternarrative: They just want confirmation that they were right all along.

“They’re broadcasting to an audience that already believes or feels a certain way about a politician, so, often, when [the truth of the alteration] comes to light, people just don’t care,” said Becca Lewis, a researcher at Stanford University who studies online extremism and media manipulation. “They say ‘it could have been true’ or ‘nonetheless, it reflects who the person really is.’ There’s a shared form of apathy in some cases for the fact that it was manipulated at all."

Anyone can go viral, build an audience or score political points by altering an image or making a meme — and doing so is not only accepted, but encouraged, by the incentive structures of the modern Web. Instead of seeing a single mailer, an online viewer can see a new image practically every minute, distributed freely and anonymously through insular and like-minded communities that constantly reinforce their own ideas.

Why do people share them? Because, researchers said, they work. The opponents of someone who shared a fake image, as in Gosar’s case, might call them out. But those reactions only serve to spread the message further, helping seed the idea on an immediate and massive scale.

There are mercenary reasons for people to create altered photos and convincing fakes, from building a fan base to making money to getting the attention of a political leader — even Trump himself. But much of the energy also stems from the polarized desire to sway political narratives and outdo the other side. It’s “a battlefield mentality,” Linvill said. “People create these memes to prove their cause is the just one."

The fakes highlight how deception from homegrown American sources may be just as destructive, if not more so, than interference from abroad. And they underline how a manipulator doesn’t need the full might of sophisticated technology to fool an online crowd: A crude photo edit, or even just a false caption on a video, can often do the trick.

Such was the case of an altered video of Pelosi from May, in which her speech was deliberately slowed and distorted in a way that made her sound impaired. The video touched off a firestorm over the way altered footage could be used to undermine opponents and distort the truth. But eight months later, the video — which Pelosi’s deputy chief of staff Drew Hammill has called “sexist trash” — remains widely available online.

Debunking such images and video can often have limited practical value, and a fact check’s virality very rarely outpaces that of the original fake. Even reporting on the images — in stories like, well, this one — can help fuel their spread.

Some of the edits are designed as eyebrow-raising parodies, the kind of satire long protected by the First Amendment. But others are deployed largely to sow uncertainty or malign one’s perceived enemies, and in an era of increasing political polarization, it is often just a challenge figuring out which is which. Video appearances are inaccurately captioned or falsified altogether. Images are skewed, mislabeled or misconstrued. Speech is clipped out of context or dramatically slowed to make one’s words sound stilted and slurred.

On Thursday, Rep. Elise Stefanik (R-N.Y.) tweeted a fundraising appeal featuring an altered, rust-colored image of Pelosi — her face so harshly sharpened it deepened every line into a craggy hollow, practically vacuum-sealing her flaws.

After Trump retweeted the image, The Washington Post wrote a story noting its “unnaturally high contrast,” and Stefanik thanked The Post for “advertising” her appeal. “Was an embarrassment for you. But great fundraising for #TeamElise,” she tweeted, alongside an emoji of a winged stack of dollar bills.

Such an edit, Stefanik’s supporters said, was par for the course: What about the photos last year on Pelosi’s Twitter account of Trump’s heavily sallowed frown or Senate Majority Leader Mitch McConnell’s dark vignette? Some compared the Pelosi image to an Instagram filter: A fact of life in an online world where photos are presumed edited until proved untouched.

Stefanik herself had recently been forced to respond to her own distortion. In November, thousands of Twitter accounts retweeted a photo manipulated to show her extending a middle finger following the impeachment-inquiry testimony of Marie Yovanovitch, the former ambassador to Ukraine. The image was portrayed as clear proof of Republican impudence: Her congressional district “deserves better than this childish loser,” one viral tweet declared.

The altered photo had been circulated by a “Leftist Twitter mob,” she later tweeted, pointing to a telltale clue in the altered photo: its magenta nail polish. “I haven’t had time for a manicure in weeks!” she said.

Some clips don’t need to doctor anything to achieve a corrosive effect. A 13-minute monologue from Biden earlier this month in New Hampshire, on the serious need to address America’s cultural legacy of domestic violence, was trimmed of all context to a 19-second racist sound bite, which then exploded across the Web.

This style of digital fiction long predates the Trump years. Obama’s face has been darkened, morphed with Joker paint and cropped to reveal devil horns. Warren has been falsely shown dancing in a “nude pagan ritual.” Hillary Clinton’s head bobs and “hand posture” were put forth as proof, in a YouTube montage in 2016, that she was hiding Parkinson’s disease: The video — which the National Parkinson Foundation says “does not in any way” support the diagnosis — has been viewed more than 5 million times.

But researchers say they’ve seen a rapid acceleration of such fakes during the Trump presidency, possibly because Trump has proved one of their most popular distributors. In September, Trump tweeted a dancing video of Rep. Ilhan Omar (D-Minn.) that had been falsely labeled as her partying “on the anniversary of 9/11.” Her tweet correcting the record — and decrying “lies that put my life at risk” — was retweeted 9,000 times, or roughly 5,000 times less than Trump’s original tweet. (The pro-Trump commentator who originally posted it later thanked Trump for the share.)

Part of that growth, the researchers say, may also stem from the increasing technical ease with which anyone can create and mass-share a doctored image, as well as the social media giants’ reluctance or unwillingness to get involved. A doctored image of Biden appearing to grab a woman’s chest, posted onto the “America’s Conservative Voice” Facebook page in 2017, has been shared more than 250,000 times and was still receiving comments — asserting the image is real and “conveniently NOT reported on by the … Fake News Media” — as recently as this month.

If fake images lead Americans to lose faith in what they see online, that could have consequences for the spread of legitimate information, too. Researchers Bobby Chesney and Danielle Citron, who labeled this effect “the liar’s dividend,” worry politicians could weaponize that doubt against their own scandals or truths inconvenient to their cause. Trump, who calls reports critical of him fake, has said the “Access Hollywood” video that recorded his boasts of assaulting women was doctored. (It was not.)

The expansion of edited media has in some cases made it harder to tell the difference between a deliberate alteration and an accident. In November, Fox News’s Facebook account posted a live video of Sen. Kamala D. Harris’s (D-Calif.) speech to a crowd in Muscatine, Iowa, with audio that seemed inhumanly garbled. Some commenters wrongly said it was clear proof that she was drunk.

The truth, as both Fox News and Harris officials would explain, was that technical difficulties in the Iowa bar’s sound system had oddly modulated her voice. But viewers of opposing stripes in the comment section saw only vindication. For some, the video proved the depth of Fox News’s distortion; for others, it proved Harris was as untrustworthy as they’d already presumed she was.

Often, the fake images and videos are only a few clicks away from being disproved. The fake photo Gosar tweeted of Obama’s fictional handshake with Iranian President Hassan Rouhani, which was sent to his 32,000 followers and retweeted more than 6,000 times, was first sighted online on the website of Al Shaab, the official newspaper of an Islamist political party in Egypt, according to a reverse-image search.

The doctored image gained new traction in recent years on American conservative blogs, message boards and ad campaigns, including in 2015, when the fake was beamed to TV screens across Wisconsin as part of a campaign boosting Johnson. The image was widely debunked by fact-checkers before the vote. (Johnson was reelected.)

Gosar’s tweet was panned by Democrats, including Rep. David N. Cicilline (R.I.), who tweeted “this is exactly the kind of thing Republicans are going to do in 2020 to hold onto power."

But Nina Jankowicz, a fellow studying disinformation at the Wilson Center, a Washington think tank, said the long-term effects of elected leaders sharing warped views of reality extend beyond any one contest.

“People are going to trust their politicians less, they’re going to trust institutions less and they’re not going to want to participate in civil discourse,” Jankowicz said. “Without that participation, our democracy just doesn’t work."