Gun-control activist David Hogg now wants to stop elected politicians from speaking to the National Rifle Association. In a tweet Sunday, Mr. Hogg noted that he had signed a MoveOn.org petition to “Demand Mike Pence cancel his speech to the NRA convention on May 4.” The tweet to Mr. Hogg’s 777,000 Twitter followers links to a petition that makes a similar demand of Nick Ayers, the vice president’s chief of staff. It was unclear whether the threat from Mr. Hogg’s followers would discourage the vice president from going to the Dallas meeting.