Voters have approved an initiative to tighten gun background checks in Nevada.

Backers of the initiative, Question 1 on the ballot, said they wanted to close what they called a loophole in current law by requiring background checks through a licensed gun dealer when most firearms change hands — including personal and online sales.

Opponents said the initiative a threat to Second Amendment gun rights that would cost law-abiding gun owners time and money.

Gun control was on the ballot in four states, including California, which already has some of the nation’s toughest gun-related laws. Voters there approved a measure that will outlaw possession of large-capacity ammunition magazines, require permits to buy ammunition and extend California’s unique program that allows authorities to seize firearms from owners who bought guns legally but are no longer allowed to own them.

Washington state approved a ballot measure that will allow judges to issue orders temporarily seizing guns from individuals who are deemed a threat.

In Maine and Nevada, a group founded by former New York City Mayor Michael Bloomberg spent millions promoting ballot measures that would require background checks on nearly all gun sales and transfers.