Senator Ed Markey went on CNN earlier this month and appeared to break major news in the investigation of the Trump campaign’s ties with Russia. “There are very strong allegations the Russians had relationships with people inside of the Trump campaign,” the Massachusetts Democrat said. “In fact, subpoenas have now been issued in northern Virginia with regard to General Flynn and General Flynn’s associates. A grand jury has been empaneled up in New York.” While it was known that federal prosecutors in Virginia had subpoenaed associates of Michael Flynn, Trump’s former national security advisor, the grand jury investigation was news to political reporters who were watching.

Fake news, it turned out. Pressed later for details, Markey’s office revealed the source of the bad information:

A Markey aide says that the Senator was referring to reports on Louise Mensch's blog and Palmer Report (seriously) https://t.co/4U8EEg3yKn — Jon Swaine (@jonswaine) May 10, 2017

Louise Mensch is a British journalist, but only her Britishness is unquestioned; whether she can rightly be called a journalist is up for debate, for she has become a chief promoter of Russia-related conjecture online, principally on Twitter. The Palmer Report is a fellow-traveler.

Markey’s mistake was the latest and perhaps most prominent example of the rise of conspiracy-mongering on the left, prompting some to worry that liberals are heading into the same fever swamps that have swallowed up the Republican Party. “Mensch and The Palmer Report are part of a disturbing emerging trend,” the New Republic’s Sarah Jones wrote after the Markey incident. “Liberals desperate to believe that the right conspiracy will take down Donald Trump promote their own purveyors of fake news.”

The left ought to be concerned about this trend, but some have gone so far as to apply a false equivalence to conspiracy-mongering. The Russia theories haven’t taken hold among Democrats in nearly the same way that countless right-wing theories—like those about Barack Obama or Seth Rich—have gripped the Republican imagination. That’s because the two parties are fundamentally different: Only one of them acts responsibly when faced with politically convenient, but obviously fantastic, stories.