What social media app has the biggest political influence right now? Not Facebook. Not Twitter.

According to Quartz, in the African country of Tanzania at least, it’s WhatsApp:

Things are a bit different in Africa, where the mobile phone has changed not only the way people communicate but the very nature of their financial lives. In Tanzania, it is WhatsApp, the mobile instant-messaging app that boasts 900 million users around the world, that has transformed political communication in the country.

A general election campaign is underway in the country, pitting presidential candidates John Magufuli from the ruling party Chama Cha Mapinduzi (CCM) and Edward Lowassa representing the opposition coalition Ukawa. The vote for a new president and parliament will take place on Oct. 25.

“WhatsApp is a preferred tool of choice for propaganda, mudslinging, and negative messaging,” a digital media strategist tells Quartz, requesting anonymity in order to openly discuss how the platform is being used by political parties. The app is the go-to platform for “messaging you can’t say yourself that surrogates can do for you,” the strategist adds. In this way, WhatsApp networks are similar to Super PACs in the US, the officially unaffiliated groups that nonetheless raise money and cater directly for their chosen candidates’ whims.

“A lot of feature phones have Whatsapp pre-installed,” Maxence Melo, co-founder of Jamii Forums, one of Tanzania’s most popular online platforms, tells Quartz. “People do not have the technical know-how to install apps. So its easy [because] of these feature phones, Chinese phones, to have WhatsApp.”

Additionally, mobile carriers are now offering cheap on-demand data bundles that bolster the popularity of WhatsApp. A user can buy 60MB worth of data for up to a week for as little as Tshs2,000 ($1). Because of this, Melo estimates that there could be as many as 8 million people using the app. That’s a lot more than the country’s 2.5 million registered Facebook users.

Additionally, WhatsApp is a way for parties to test negative messages they want to break into the broader news cycle. “It becomes a catalyst, a driver of what one should do next,” the digital strategist explains. Once something is seen to resonate on the relatively closed network of WhatsApp, operatives will push it to other social media platforms. “If it works on Whatsapp then it’ll go on Twitter,” the strategist says.

So… pre-installed… Chinese phones… cheap and easy… used as testing ground… political messaging strategies.