According to a Republican source who tracks television ad buys in Pennsylvania, the Democratic Senatorical Campaign Committee is retreating--at least temporarily--from Joe Sestak's campaign against Pat Toomey.

The DSCC has spent nearly $3 million on television advertising in Pennsylvania over the past five weeks, renewing its ad buy each Monday "like clockwork," according to our GOP source. Airtime needs to be booked by 5p.m. each Monday in order for ads to air in the coming week. But today the DSCC didn't buy any ads. That means that as of tomorrow, the DSCC will go off the airwaves. The DSCC has not yet responded to a request from TWS one hour ago to confirm or deny that they've gone off the air in Pennsylvania.

"They're either pulling out or they have a money crunch," says the GOP source, who notes that Democrats now need to spend money on Senate races in states like West Virginia. According to the RealClearPolitics average of polls, Toomey's lead over Sestak has been fairly stable during the DSCC's 5-week ad campaign, with the Republican holding anywhere from a 6 to 8 point lead over the Democrat. Toomey's current lead over Sestak in the RCP average of polls is 6.5 points.

But it's not out of the question that the DSCC could decide to pour money into the race later if Sestak closes the gap. "They could come back," says the GOP source. "But it's just very rare that you would start like this"--spending $3 million over five weeks--and then go silent.

According to our source, as of late this afternoon, the only groups scheduled to run Senate campaign right now on Pennsylvania TV are the Toomey and Sestak campaigns, as well as two independent groups working to defeat Sestak, the Club for Growth and American Crossroads.