Sen. Arlen Specter urged “mainstream Americans” to turn out in droves to prevent a total rejection of moderate challengers and incumbents. “Mainstream Americans must march to the polls this November to express themselves forcefully to stop extremists financed by undisclosed contributors from stifling our democracy,” the Pennsylvania Democrat said in a floor statement Tuesday morning. Specter, who left the GOP for the Democratic Party last year after determining he could not win the Republican primary in his state, noted this year’s primary process has been particularly devastating to moderates. Specter, a moderate, is one of three incumbent Senators to lose...