Shortly after Vice President Kamala Harris chose Minnesota Gov. Tim Walz as her running mate, the Trump-Vance campaign called into question claims he previously made about his military service, opening a broadside against the U.S. Navy veteran that some Republicans now liken to “sort of a Swift Boat 2.0 attack.” It is a reference to the highly contentious playbook the GOP used against Sen. John Kerry when the Massachusetts Democrat ran for the White House in 2004, in large part on his record as a junior naval officer in Vietnam. Those character attacks, widely panned at the time, called into...