WASHINGTON — As Democrats on Capitol Hill staged a sit-in in the House chamber seeking a vote on gun policy measures, the ACLU sent a letter to senators opposing the latest compromise measure being considered.

The measure — proposed by Republican Sen. Susan Collins and backed by the Democrats' leader in that chamber, Sen. Harry Reid — is a more narrow version of the "terrorism watchlist" gun ban measure proposed by Sen. Dianne Feinstein that was rejected earlier in the week.

Collins' measure utilizes the No-Fly List and the Selectee List as the basis for restricting gun purchases — which would include a far narrower group of individuals than the broad terrorism watchlist used in Feinstein's original proposal — but that change was not enough to lead the ACLU to support the measure.

"The ACLU strongly urges you to vote against the Collins Amendment because it uses the error-prone and unfair watchlist system, along with vague and overbroad terms, as a predicate for a proceeding to deny a firearms permit," the ACLU officials wrote in the letter. "The Collins Amendment relies on both the No Fly List, by codifying its criteria, and the Selectee List, by direct reference."

The ACLU letter goes on to detail concerns with the due process protections in the amendment, stating that the amendment would provide even fewer protections than the amendment proposed by Sen. John Cornyn earlier this week — which the ACLU also opposed.

Finally, the organization warns that the Collins amendment "would impose a notification requirement that could result in a new 'watchlist' broader than any that currently exists." This is so, the ACLU argues, because the amendment would require officials "to be informed of each application for a firearm by any person who has been on the master watchlist at any point over the past five years—even if the person has been cleared of any wrongdoing, the investigation was otherwise closed, or the person was long ago removed from the list."