Democrats’ recent efforts to secure votes on gun control measures in the wake of the mass shooting in Orlando, Florida have stoked criticism not just from Republican leadership but from some on the left.

The American Civil Liberties Union on Wednesday sent a letter to senators laying out its opposition to the latest legislation up for consideration, a proposal by Sen. Susan Collins (R-ME) that would bar anyone on certain federal government watchlists from buying guns.

“The ACLU strongly urges you to vote against the Collins Amendment because it uses the errorprone and unfair watchlist system, along with vague and overbroad terms, as a predicate for a proceeding to deny a firearms permit,” ACLU legislative leaders wrote, saying reliance on the lists “would open the door to arbitrary and discriminatory government action.”

Collins’ amendment, which is backed by Senate Minority Leader Harry Reid (D-NV), targets those on the No Fly List and the Selectee List, a secret list which selects certain passengers for additional inspection at U.S. airports. This is a narrower frame than the measure proposed by Sen. Dianne Feinstein (C-DA) earlier this week, which targeted anyone on the broader terrorist watchlist. Feinstein’s amendment was defeated on Tuesday, along with another offered up by Sen. John Cornyn (R-TX).

The ACLU opposed both measures for relying on a watchlist system that has in the past swept up innocent individuals, is kept secret, and offers no “meaningful process” for people to “correct government error and clear their names.”

Glenn Greenwald of The Intercept, an investigative national security news site, voiced similar concerns in a Tuesday piece cautioning against the likelihood of racial profiling and the overly broad language in these amendments.

“Democrats, in unison, are actually arguing that the U.S. government must constrain people whom they are now calling ‘potential terrorists,’” Greenwald wrote. “Just spend a moment pondering how creepy and Orwellian that phrase is in the context of government designations.”

As this debate raged, Democrats took to the House floor to hold a sit-on over Republican leaders’ refusal to call votes on a pair of gun control measures before the chamber left for recess. Though the House adjourned until July 5 early Thursday morning, Democratic leaders remained on the House floor in protest.

Read the ACLU’s full letter below.