Pete Buttigieg’s recent popularity surge in Iowa and New Hampshire has many speculating that President Trump could face a Democratic ticket in 2020 that he doesn’t want and fears the most — the South Bend, Ind., mayor and another candidate recently in the spotlight, Rep. Tulsi Gabbard of Hawaii. Despite what the “experts” might say, it’s certainly possible. Like weather forecasters and economists, political pundits often are proven embarrassingly wrong. Voters from both parties have a tendency to recalibrate rather quickly, and quite unexpectedly, toward candidates they believe actually might have a chance of winning. In 2004, for example, Democratic...