"It really is domestic intelligence interference in our election," Greenwald said on Hill.TV's "Rising."
"It's interference by the CIA and by homeland security and by related agencies in our domestic election, which is infinitely more threatening to our democracy than whatever mischief Russian agencies are primitively doing on Facebook and Twitter."
Greenwald's assessment was in response to Democratic presidential candidate Sen. Bernie Sanders (I-Vt.) acknowledging last week that he was briefed by U.S. intelligence officials about Russian attempts to interfere in the 2020 elections.
Sanders said he was briefed after The Washington Post reported that Russia has sought to help Sanders's presidential campaign.
Greenwald said the "theme or narrative they've settled on" that Russia is looking to help Sanders to nominate the "weakest candidate" against President Trump is "completely moronic."