The election campaign between Donald Trump and Joe Biden is so far largely a personal showdown, but their policy ideas matter to the country’s prosperity. Mr. Trump can claim the better record on the economy, thanks in part to his 2017 tax reform, so it’s too bad he’s now floating special-favor tax ideas like exempting worker tips from federal taxes. “For those hotel workers and people that get tips, you’re going to be very happy,” Mr. Trump said on a recent swing through Nevada, “because when I get to office, we are going to not charge taxes on tips.” He...