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 this year. He lost in May to liberal Democratic Rep. Joe Sestak.

“The Senate is a vastly different place than it was when I was elected in 1980,” Specter said.

In a thinly veiled charge, Specter took after one of his colleagues, Sen. Jim DeMint (R-S.C.), whom he blamed for trying to impose a conservative purity test on Republicans.

“Within days of the start of the Obama administration, before the ink was dry on his oath of office, Republicans openly bragged about plans to ‘break him and engineer his Waterloo,” Specter said of DeMint’s statements during the health care debate. “Announcing that ideological purity was more important than obtaining a majority, the prevailing Republican motto was, ‘We’d rather have 30 Marco Rubios in the Senate than 50 Arlen Specters.'”

Rubio, a conservative Republican, is running for Senate in Florida. His surge in the polls prompted moderate Gov. Charlie Crist to abandon plans to run for that seat as a Republican. Crist is running as an Independent.

“Pundits are saying this November our nation will be at the crossroads. I believe it is more like a cloverleaf,” Specter said. “If activated and motivated to vote, mainstream voters can steer America to sensible centrism.”