In Nevada, where a school shooting school shooting today left two dead--including the shooter--and two injured, the governor vetoed a bill last June that would have strengthened background check laws.

Gov. Brian Sandoval, a Republican, rejected the measure despite an advertising blitz by the pro gun control group funded largely by New York City Mayor Michael Bloomberg, Mayors Against Illegal Guns.

The pro-gun control group advertised early and often Las Vegas this year, according to a search on Political Ad Sleuth, the Sunlight tool that tracks spending on political ads. Ads began in March and the most recent one aired ...