Mass shootings, including those in Las Vegas and Parkland, Fla., have thrust the issue of gun control into midterm races that will determine whether Republicans hold control of Congress and maintain dominance in state capitals. Ads promoting stricter gun regulations have aired 102,636 times across the country this year—a 22-fold increase from four years ago, according to a Wall Street Journal analysis of Kantar Media/CMAG data.

From behind the wheel, steering toward the Las Vegas Strip, the Democratic candidate for Nevada governor says in one of his campaign ads, “I’ll never forget driving down after the shooting.”

Steve Sisolak, now the commissioner of Clark County, which includes Las Vegas, was talking about Oct. 1, 2017, when a gunman killed 58 concertgoers from a hotel room perch. “When I’m governor, we’re going to ban assault rifles, bump stocks, silencers,” he says in the ad. “We need to take action.”

State Attorney General Adam Laxalt, the Republican challenging Mr. Sisolak, opposes gun-control measures and has been endorsed by the National Rifle Association. He hasn’t mentioned guns in any of his ads.

Americans are more likely to see political ads promoting gun control than ones opposing stricter regulations, a reversal from four years ago. In Nevada and Florida, the turnabout is striking: Those states went from zero pro-gun control ads in 2014 to more than 45,000 this year.