Last week, President Donald Trump reiterated his pledge to eliminate a little-known tax law that bars charities, including churches, from endorsing political candidates. It’s called the Johnson Amendment, and the IRS has rarely enforced it. Proponents say it maintains an important barrier between church and state. Those who oppose it say their free-speech rights are being infringed. Tuesday, we’re talking about the history of the Johnson Amendment and what its erasure could mean for the