This month, Stephen Yaxley-Lennon, known as Tommy Robinson, a British far-right activist and former leader of the extremist English Defense League who is also running for the European Parliament, was hit by two milkshakes in one day at the start of the recent spate.

Danyaal Mahmud, who first doused Mr. Robinson, told the newspaper The Observer that he had taken the action after becoming offended by what Mr. Robinson had been saying to him. He said he had used a milkshake simply because it had been in his hand at the time. The moment was captured on video and quickly spread on social media. A day later, in a different city in northern England, Mr. Robinson was again splattered with milkshake.

Carl Benjamin, a member of the right-wing U.K. Independence Party who is under investigation about a comment he made on Twitter regarding raping a female Labour lawmaker, was later targeted. He has been hit with four separate milkshake attacks so far.

Other attacks on the far right have been encouraged on social media with the hashtag #SplashTheFash. The practice has prompted an angry backlash from some rightists.

“Just to be clear, anyone that comes at me with a milkshake will need the straw to eat their meals for the next few months,” Mark Meechan, another UKIP candidate, wrote on Twitter this month.

The attacks have also caught the eye of the police. In preparation for a weekend rally for Mr. Farage in Edinburgh, officers have asked McDonald’s to briefly stop selling milkshakes on Saturday, a spokeswoman for the fast food chain said.

But why milkshakes?

Kevin Featherstone, a professor of European politics at the London School of Economics, said in an email that the strong visual impact was important.