The WhatsApp and Facebook app icons on a smartphone. (Patrick Sison/AP)

FACEBOOK HAS one advantage in its war against the misinformation rampant on its platform: The company can see the falsehoods. The same is not true on WhatsApp, the chat software owned by Facebook, where messages are encrypted end-to-end — and where political missives promoting conspiracy theories are spreading to users in Brazil ahead of the country’s presidential runoff election on Sunday.

WhatsApp is immensely popular in Brazil. It is also filled with lies. National outlet Folha de São Paulo reported last week that companies purchased millions of dollars in messaging packages to blitz users with propaganda supporting far-right candidate Jair Bolsonaro. Researchers who analyzed the 50 most widely shared WhatsApp images in Brazilian chat groups open to the public discovered that more than half were misleading or false. Some messages smeared Mr. Bolsonaro’s opponent as a communist. False information also has smeared Mr. Bolsonaro, such as with an accusation that he faked his stabbing at a campaign event.

WhatsApp is fighting the disinformation, but in many ways the app is its own greatest enemy. WhatsApp markets itself as a place for private conversations, lending even its largest groups an intimacy that makes users trust what they read. Yet because those groups can contain 256 people, and users can also individually message 256 contacts at once, bad actors can reach thousands of people with only a few taps of a phone screen. All the while, end-to-end encryption prevents anyone, even WhatsApp’s own staff, from seeing which messages travel the furthest and the fastest.

There’s another wrinkle: Many mobile plans in Brazil include Internet access only to Facebook and WhatsApp, which means users may struggle to consult news sites for clarification on fishy stories.

Researchers have suggested fixes to salvage the integrity of Sunday’s vote, including further lowering the limit on the number of times a message can be forwarded, as the company did in India after inflammatory rumors on the app led to mob lynchings during the summer, as well as limiting the number of contacts a user can reach at once. Researchers also want WhatsApp to temporarily restrict the size of new groups in Brazil.

But introducing these changes so late in the game could end up slowing down debunking efforts as disinformation spreads apace. A worldwide rethinking of WhatsApp that would decrease the reach of users or decrypt groups above a certain size could come with drawbacks, too: If WhatsApp abandoned its defining features, another company might step in. And in repressive countries, WhatsApp might offer civil society the ability to organize where the government cannot spot dissent and shut it down.

WhatsApp needs to acknowledge that it is much more than the private messaging platform its executives promote. It is a publisher, just like its parent company and Facebook’s peers. That means taking responsibility for the disinformation and violence it facilitates. There is no easy answer. But there might be a hard one, and WhatsApp needs to be searching for it.