Internal GOP polling conducted last week showed Castle with a margin of nearly 20 points over O’Donnell.

But after Murkowski’s loss, there is a desire among Republican operatives to be safe rather than sorry…

A senior Republican operative granted anonymity to speak candidly about the race said Murkowski’s defeat was the best thing that could have happened to Castle. “After Alaska, Castle knew he would be next if he didn’t run a real campaign – including defining O’Donnell,” the source said.

Castle, like Murkowski, was counseled by the national party to go on the attack immediately against O’Donnell, taking advantage of the $2.6 million he had in the bank at the end of July (O’Donnell had $70,000). Unlike Murkowski, Castle listened. He launched radio ads that describe O’Donnell as a “financial disaster.” Tough TV spots are on the way, according to knowledgeable insiders.