Specter himself is a two-time cancer survivor. He was diagnosed and underwent treatment for Hodgkin's disease last year. He recently published a book "Never Give In: Battling Cancer in the Senate" about dealing with the disease after his initial diagnosis in February 2005.
Republican National Committee Chairman Michael Steele said the move is to further Specter's own political interests, adding in a CNN interview that the Republican party looks forward to defeating him in 2010, if the Democrats don't do it first.
After a weekly caucus lunch for Republicans, at which they learned of Specter's defection and briefly saw their former colleague, Senate Republican leaders admitted they were unhappy losing a member, but argued that it has nothing to do with the national Republican Party rejecting moderates.
This is about local Pennsylvania politics and Specter's desire for "political self-preservation," they said.
"Well, obviously, we are not happy that Sen. Specter has decided to become a Democrat. He visited with me in my office late yesterday afternoon and told me quite candidly that he'd been informed by his pollster that it would be impossible for him to be reelected in Pennsylvania as a Republican because he could not win the primary; and he was also informed by his pollster that he could not get elected as an independent, and indicated that he had decided to become a Democrat," Senate Minority Leader Mitch McConnell, R-Ky., said.
McConnell said Democrats will have a dangerous capacity if they can reach 60 votes, which they may do if and when Al Franken takes the Minnesota Senate seat.
That scenario has looked more likely with court decisions over the contested Senate race in Minnesota, but Republicans are sure to drag out their challenges on behalf of former Sen. Norm Coleman, who has slightly fewer votes than Franken in that race.
"It certainly sets up the potential -- the potential -- for the majority, if it chooses to, to run roughshod over the majority -- over the minority, to eliminate checks and balances and the kind of restraint that Americans have historically wanted from their government," McConnell warned.
McConnell dismissed the impression given by Specter's defection that moderates don't feel comfortable as Republicans.
"I reject that out of hand," he said. "We have moderates in our conference who have an enormous amount of influence. I can tell you, and you know this, because you're around here all the time, there's a good deal of difference between a senator from Maine and Mississippi. But we have a broad party."
But discomfort had everything to do with Specter's decision as he decried Republicans in Pennsylvania for going too far to the right and rattled off several recent House races in which moderate Republicans were defeated in primaries by more conservative Republicans.
Cornyn explained away Specter's decision: "[Specter] was very candid to acknowledge that this was simply nothing more, nothing less than political self-preservation. ... So his only options were to leave the Senate or to switch parties, since he was determined or he was convinced he could not win as an independent."