The National Basketball Association, alarmed by the death toll from shootings across the country, is stepping into the polarizing debate over guns, regulation and the Second Amendment with an advertising campaign in partnership with one of the nation’s most aggressive advocates of stricter limits on firearm sales.

The first ads, timed to reach millions of basketball fans during a series of marquee games on Christmas Day, focus on shooting victims and contain no policy recommendations. The words “gun control” are…