This decision, to me, comes down to the ability to secure rents in the public arena. You can split these rents into current (i.e. this Congress) and future (i.e. future Congresses) flows. Initially, before thinking about any of this, I'd presume that changing parties would help you secure rents in the present, but maybe with a tradeoff. This may come at the expense of angering your constituency, but the more I think about it, that may not be a large issue at all. Do constituents elect Republicans and Democrats, or individuals? If it's the former, they might be upset-- but if it's the latter, and a change in party generates more political power and more pork, all the better in the eyes of the voter.
Of course, there's a future rent flow to consider as well-- and that seems to be the underlying issue for Specter.
These are big moments in Congressional history; the Jeffords effect has received a fair amount of attention in the journal world.
The senator, who has represented Pennsylvania in the upper chamber since 1980, said he was "anxious" to stay in the Senate -- and he did not want to face a Republican primary in order to keep his seat next year.
"I was unwilling to subject my 29-year record in the U.S. Senate to the Pennsylvania Republican primary electorate," he said. "But I am pleased to run in the primary on the Democratic ticket and am ready, willing and anxious to take on all comers in the general election."
Polls suggested Specter would face a stiff primary challenge from Rep. Pat Toomey, who falls to his right on the political spectrum. Toomey nearly defeated Specter in the Pennsylvania GOP Senate primary in 2004.