Democrats continue to celebrate one of the most lopsided midterm election victories in modern U.S. history, routing their Republican rivals to gain control of the U.S. House, U.S. Senate and statehouses across the country.

In the Senate, Democrats flipped seven seats and now hold 54 to the Republican Party's 44. Two seats — nail-biting contests in Alaska and Mississippi — remain undeclared.

To do so, Democrats won a series o...