Mellie's bid for Virginia senator goes right to a general election, though the runoff process would have involved primaries first. And during these primaries she likely would have been challenged by another Republican, who would have lambasted her slight ties to Virginia and questioned whether the President chose Susan Ross as Vice President in order to open up a senate seat geographically contiguous to D.C. where his wife could conceivably run for office. Pretending there was no primary offered the writers a way to ignore these problems.