The Justice Department filed a lawsuit against Denver on Tuesday over the Colorado city’s 37-year-old ban on so-called “assault weapons,” including constitutionally protected semiautomatic rifles, after the Democratic mayor signaled defiance.

The court filing alleges Denver’s firearms ordinance features “politically charged rhetoric” regarding its use of the term “assault weapon.” AR-15-style rifles fall under this category, per the law, despite being the most popular rifle in the nation.