Robert F. Kennedy Jr.’s endorsement of former President Donald Trump is a classic example of Ronald Reagan’s famous explanation for the change in his political allegiance: “I didn’t leave the Democratic Party, the Democratic Party left me.” And it has the power to tip the election in Trump’s favor, especially in the swing states that will decide it. Endorsements, while highly coveted by politicians, don’t usually change election outcomes, but this one feels different. Kennedy, a blue-blood Democrat from the nation’s most famous political dynasty, is speaking to many fellow “former Democrats” — especially newly activated moms like me. In...