The Justice Department sued Virginia and California on Wednesday over their newly enacted restrictions on guns, arguing they are unconstitutional under the Second Amendment. The suits are part of a broader effort to roll back measures intended to limit the use of firearms at a state and national level.

Twin lawsuits filed by the administration target a ban by Virginia on the sale of semiautomatic assault rifles and a ban by California on the sale of Glock and Glock-style handguns, which can easily be converted into a weapon that can fire at the pace of a machine pistol. Both laws went into effect on Wednesday, though in Virginia state judges have blocked it in some jurisdictions.

Since President Trump assumed office and ordered a rollback of regulations his predecessor had put into place after years of debate on gun control and a string of devastating mass shootings, the fight over gun control has largely moved to states and localities. In May, the administration sued Denver over its ban on possessing semiautomatic rifles.

The most recent lawsuits come a day after the Supreme Court said it would hear challenges to local bans on semiautomatic assault rifles in its next term, raising the possibility that the conservative supermajority would strike down those laws as unconstitutional.

The Virginia law has been faced resistance. Several sheriffs and local prosecutors have refused to enforce the ban, and shoppers interested in buying assault rifles have lined up outside of gun stores, hoping to buy before the law was set to go into effect on Wednesday. Gun owners and the National Rifle Association also have succeeded in blocking the law’s enforcement in some local jurisdictions.

The California law has also faced challenges from the N.R.A. and other gun-rights groups. In its suit, the Justice Department criticized the state’s reasoning for focusing on Glock-style handguns.

“Apparently, the state believes it can outlaw perfectly legal and safe firearms if those firearms can be converted into illegal firearms,” it said. “This is not the law.”

In response, representatives for the Democratic governors of both states and Virginia’s attorney general said they would defend what they called “common sense” laws, asserting that the measures were intended to protect their residents.

Diana Crofts-Pelayo, a spokeswoman for Gov. Gavin Newsom of California, pointed to what she called the state’s low rate of gun deaths and “historically low crime rates.”

“California has proven that strong, evidence-based gun safety measures can reduce gun violence while respecting the rights of responsible gun owners,” Ms. Crofts-Pelayo said.