Virginians cast ballots Tuesday on a proposed congressional map that would aggressively gerrymander the state in the Democrats’ favor, giving the party as many as four extra U.S. House seats.

Virginia Democrats – who currently hold six of the state’s 11 House seats – stand to eliminate all but one Republican district if a critical redistricting referendum goes their way.

Virginia voters approved a referendum on Tuesday to redraw the state’s congressional map, according to The Associated Press, allowing Democrats to flip as many as four Republican-held seats in the U.S...