U.S. Sen. Arlen Specter, now a Democrat representing Pennsylvania, shocked the political class in Washington on Tuesday when he announced that after 29 years in the Senate as a Republican that he would switch parties and run as a Democrat for re-election next year.

Immediately, the national media read his statement verbatim on air and ascribed motives to his decision. They asked, "How can a moderate politician remain a Republican if they feel that they have to switch parties to win re-election?"

The answer is that for Mr. Specter, this decision was selfish, unprincipled and limited to his own self-preservation.

But don't just take my word for it.

At a hastily arranged press conference in the U.S. Capitol on Tuesday afternoon, Mr. Specter was asked by CNN's Dana Bash why he was switching parties after stating unequivocally that he would not switch parties merely a month earlier. Here is his response:

"The decision has been reached gradually as I have traveled the state over the past several months. I got my own poll results back last week and consulted with my campaign managers and had a long discussion with Joan and my son Shanin over the weekend and came to a decision this past weekend."

It's that simple. He got bad poll results. He threw away 29 years of representing the state of Pennsylvania as a Republican in the greatest deliberative body in the world because he got bad poll results.

On March 17, The Hill newspaper reported that Mr. Specter had ruled out running for re-election as a Democrat but might consider running as an Independent as a last resort. At the time, Mr. Specter said, "I'm staying a Republican because I think I have a more important role to play there. I think the United States very desperately needs a two-party system and I'm afraid that we're becoming a one-party system, with Republicans becoming just a regional party."

But Mr. Specter was facing stiff resistance at home from Pennsylvania Republicans. A recent Susquehanna poll showed 66 percent of Pennsylvania GOP voters preferred a new senator, with only 26 percent saying they would support Mr. Specter. Many Republican voters back home were outraged that he broke from the party to provide one of three necessary Republican votes to ensure passage of the $787 billion stimulus bill.

At the time Mr. Specter said he voted for the stimulus bill, which every single House Republican opposed, because of his principles, accepting the political challenges that it would cause him.

"Sometimes [adhering to principles] might be too high a price to be able to stay here," he said. "When I voted on the stimulus package, I was well aware of the political peril."

Facing a 21-point deficit in the GOP primary against former Rep. Pat Toomey, according to an April Rasmussen poll, and a net unfavorable rating of 13 points, Mr. Specter put his political career ahead of his principles.

Once before, in 1965, Mr. Specter switched parties to run as a Republican for district attorney in Philadelphia.

Mr. Specter recently said, "I think each of the 41 Republican senators, in a sense -- and I don't want to overstate this -- is a national asset," he said, "because if one was gone, you'd only have 40, the Democrats would have 60, and they would control all of the mechanisms of government."

Well, soon, once Al Franken, the likely victor in the Minnesota Senate race, is seated, Democrats will control all the mechanisms of government. And Arlen Specter will be the Democrats' problem.