Midterms tend to be backlash elections, and the president's party almost always loses seats when voters return to the polls to judge his performance in office. 

But not all midterms are created equal. 

On Tuesday, Democrats comprehensively routed their Republican rivals, regaining control of the U.S. House, U.S. Senate and statehouses across the country. Along the way, they shattered a series of decades-long Republican winning...