Setting aside that this ultimatum has come, for the time being, from the Daily Kos it's extremely plausible that this type of ultimatum or something similar will come from the Party itself:
That's the bottom line.Seat Al Franken and give him his committee assignments now, or we'll block a new organizing resolution that would let you reassign Specter's previously Republican committee seats to one of your own.
Until a new organizing resolution is adopted, Specter's committee seats (Appropriations, Judiciary, Veterans' Affairs, Environment & Public Works, Special Aging) are locked in. He'll be caucusing and (sometimes, anyway) voting as a Democrat, but will be occupying Republican seats.
Actually, with the exception of Appropriations and the Judiciary I'd say: "you can have those seats" as Specter is at best unreliable either way on many of these issues.
Again, leaving aside the source, This should be an eyeopening reminder that Democrats in particular and liberals in general are all about gaining power and manipulating their way towards that goal by any means necessary rather than putting the good of the county first!
H/T to Aaron Gardner for the source material