The American Civil Liberties Union (ACLU) told a Republican congressman that he is violating the constitutional rights of his constituents by blocking them on social media — and they're threatening to sue.

The New York state affiliate of the ACLU warned Rep. Peter King (R-N.Y.) in a letter that they might pursue legal action against him.

"We just sent a letter to @RepPeteKing, demanding that he stop blocking constituents who criticize him on Facebook," the official Twitter account of the NYCLU tweeted Wednesday.

"Silencing constituents on Facebook is like ejecting them from a town hall," they added. "It strikes at the heart of the First Amendment's guarantee."

They posted two examples of critical comments on King's Facebook page that led to the authors being "banned" from accessing his web page.

"Silencing these voices is an affront to the First Amendment and to the core values of our democracy," the group wrote in their letter to King. "If you do not unban these constituents promptly, we intend to file a federal lawsuit to vindicate their rights."

King responded to The Hill, saying that he was allowed to run the Facebook account however he wanted because it was an extension of his political campaign, and constituents were free to reach him via his Twitter account, which is a government account.

He's not the only one who has been accused of acting against constituents' rights on social media. President Donald Trump lost a lawsuit in May 2018, alleging that he was acting improperly when he blocked certain constituents on Twitter.

Here's more about unconstitutional social media blocking: