Former Republican National Committee Chairman Ed Gillespie made it official Wednesday, announcing in a web video that he will challenge Democratic Sen. Mark R. Warner of Virginia this year. Mr. Gillespie said in the video that he’s running for Senate “because the American Dream is being undermined by policies that move us away from constitutional principles of limited government and personal liberty." Virginia Republicans, hungry for a victory after being shut out of the top three statewide offices for the first time in more than two decades, are mobilizing quickly to target Mr. Warner, a popular first-term incumbent and former...