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By Peter Roff Fellow, Institute for Liberty/Former Senior Political Writer, United Press International

Many years ago, when I was just knee-high to an intern, a long time veteran of Capitol Hill told me a story about a particularly obnoxious senior Democratic member of Congress for whom his staff felt little affection.

Over the years a tradition had developed where the Senator's chief Washington aide would send a bottle of aspirin to the director of the district office anytime the congressman went home for an extended period. And then the district director would send the bottle back to the D.C. chief of staff when the congressman returned to the Capitol.

I bring this up only because that was sort of my immediate reaction when I heard that longtime United States Senator Arlen Specter was announcing he would seek re-election as a Democrat and would be changing parties. And if I had a bottle of aspirin handy I would send it to Senate Majority Leader Harry Reid, 'cause Specter is his headache now.

Party labels have never meant very much to Specter. He is what he is and he does what he does. And, from a political standpoint, the decision to run for re-election as a Democrat -- when he was facing almost certain defeat at the hands of former GOP Congressman Pat Toomey in the upcoming Republican primary -- makes a lot of sense. It also creates a lot of problems for the Democrats, in Pennsylvania and nationally.

There is, for example, no place for the popular but term-limited Gov. Ed Rendell to go if Specter runs as a Democrat. And Rendell, a former national Democratic Party Chairman, isn't likely to be happy serving as Janet Napolitano's replacement in Obama's Cabinet. And there are several up-and-coming Democrats in Congress who now find their path to advancement blocked by Specter's decision.

Nationally, Specter's leap across the fence strips the Red-State Democrats in the Senate of much needed cover. With Specter, the Democrats are now one vote closer to the 60-vote filibuster-proof majority they need to theoretically render Republican input on legislation unnecessary to its passage. But that only puts more pressure on Democrats like Ben Nelson of Nebraska, Arkansas' Blanche Lincoln and David Pryor, Florida's Bill Nelson and -- in an odd way -- Reid himself, who is no doubt quite conscious of the way the Republicans used former Senate Democratic Leader Tom Daschle's national liberalism to drive a wedge between him and the more conservative electorate in his home state of South Dakota.

With Specter in the Democratic Caucus, the leftwing groups like MoveOn.org, the SEIU and the Sierra Club will have to increase their pressure and the expenditure of their resources to keep the moderates in line -- or threaten them with interparty discipline that just might push them over the fence in the other direction. Ben Nelson and Blanche Lincoln, just to name two Senators, would have a lot easier time winning re-election as Republicans than as Democrats.

Specter's switch isn't likely going to make much difference in the way he votes either. He has always put his own concerns and those of the people of his state ahead of those of his party. As one former Pennsylvania elected official told me recently:

"Arlen Specter is among the most dependable people in Congress. If he does something you like today, you can be sure by the end of the week he will do something that makes you really mad. And if he makes you really mad today, by the end of the week he will have done something that makes you glad he's in the Senate. Because that's just the way Arlen is."

And Specter knows how to drag a compromise out of the Senate leadership in exchange for his support. He's not unsure about making his demands and he's not afraid of having people mad at him -- 'cause he knows that, sooner or later, they're going to need his help on something.

At the end of the day, I bet there will be more than a few Democrats who are wishing for the halcyon days when Specter was a worry for the Republicans, not for them.