Specter is surprisingly candid in describing the purpose of this written explanation of his position on that legislation:
My vote was cast recognizing the very substantial political peril that I face. I know that there are many on the Republican political spectrum who do not like the vote. I remember, obviously, the tough primary fight I had in the year 2004. But I felt in the final analysis, given the very severe consequences which might befall the country, that my duty was to look out for the public interest and not my own personal political interest.
Specter is being proactive in addressing the electoral implications of the vote, which I described here. That's understandable, but I would not have expected him to so directly say, in effect, "you won't like what I've done, but vote for me anyway".