Donald Trump presented himself throughout the 2016 presidential campaign as the consummate negotiator and dealmaker. You need flexibility, he said, along with hugs, cajoling and some "tug and pull." Likewise, Hillary Clinton said one of her goals if she became president would be to create "a nice warm purple space" for compromise in Washington. Such talk may sound unrealistic given the gridlock and polarization that afflicts our government, yet breakthroughs...