Amid an increasingly passionate nationwide debate over gun safety, the National Rifle Assn. on Sunday rejected President Trump's call for a federal ban on rifle sales to those younger than 21, but a spokeswoman sought to play down the disagreement, suggesting Trump could change his mind.

The exchange illustrated the ways in which both allies and opponents of gun-law reform will have to maneuver gingerly when dealing with a president known for fluidity in his policy positions, especially in matters on which public opinion is at odds with the traditional views of his Republican party.