President Obama was all about hope. He leaves warning against fear. After his farewell address in Chicago, how will history judge his presidency? At 55 percent his job approval is equivalent to Ronald Reagan's at the same point, and ahead of Bill Clinton's. It is more than 20 points higher than George W. Bush's. But Donald Trump has promised to repeal the Affordable Care Act, possibly pull the U.S. out of the Paris agreement on climate change and tear up the U.S.-Iran nuclear deal. Each would undo some of President Obama's most visible achievements. What does he leave behind?