The Legality of Party Swapping

"...I am not prepared to have my 29-year record in the United States Senate decided by the Pennsylvania Republican primary electorate -- not prepared to have that record decided by that jury, the Pennsylvania Republican primary electorate..." Senator Arlen Specter, April 28, 2009

What electorate Arlen, the electorate that GOT YOU THERE for 29 years?!

I remember the Left talking about how wrong it would be to the Connecticut voters if Joe Lieberman switched parties...and how wrong it was for Zell Miller to betray the Georgians that had voted for him...

Now as a Pennsylvanian that (*look of shame*) voted for Arlen Specter as a REPUBLICAN...what am I to make of the meaning of my vote in light of this defection (welcome of not)?

In the comments yesterday, many asked the same question that I am curious about...Is a party switch in the middle of an elected term legal? Have Constitutional lawyers addressed this subject? Can Specter take my vote and keep it under false pretenses...legally?