During her 2010 U.S. House campaign, Democrat Ann Kirkpatrick of Arizona highlighted her “A” rating from the National Rifle Association. She opposed gun-control measures and regaled constituents in her largely rural district with stories of hunting with her father.

In 2018, Ms. Kirkpatrick is running to return to the House espousing a gun-control platform that is among the country’s most aggressive. She is for universal background checks and a ban on guns described as assault weapons. She disavows her longstanding position...