DES MOINES, Iowa [KCCI] — Iowa is joining a lawsuit challenging a new rule that would stop some private firearm sales. A new rule from the Bureau of Tobacco and Firearms, known as ATF, would require a federal firearms dealer license for every person who sells a gun for a profit.

The change would force some private sellers to conduct background checks on potential gun buyers. The rule is set to go into effect on May 20. The filed lawsuit says the rule is unconstitutional because it is vague and circumvents Congress.

Attorneys General in 21 states are part of the lawsuit against the new ATF rule.