Senator Lieberman's commitment to Democratic principles has been an open question for years. After serving as Al GoreAl GoreOPINION | Bolton: China is our last diplomatic hope for North Korea How the New South became a swing region Bill Maher compares Republican Party to trolls MORE's running mate, Lieberman became an ally of George W. Bush and a cheerleader for the War in Iraq. The party stood by him during a losing primary campaign in 2006, but held on to his seat by running as an "Independent Democrat."

Despite campaign assurances that he believed in Democratic principles, Joe went on to endorse John McCainJohn McCainBush biographer: Trump has moved the goalpost for civilized society White House to pressure McConnell on ObamaCare McCain: Trump needs to state difference between bigots and those fighting hate MORE and Sarah Palin and delivered a featured address at the Republican National Convention.

At the convention, another McCain/Palin stumper, Republican Senator Arlen Specter, had this to say about his good friend:

"I would like to see him vote with Republicans in September. He's practically there. That would have the consequence of giving us a Republican Senate."

Looks like Arlen has gotten his wish.

This should be a cautionary tale for Pennsylvania Democrats. Lieberman and Specter share the same politics and have had long careers defined by "independence" -- from their parties, their constituents, and their own positions.

Joe says he now feels "liberated" from his party -- and the positions he campaigned on. What happens if Arlen is liberated by never again having to face the voters?

Arlen Specter is our Joe Lieberman, and if Pennsylvania progressives elect him they will regret it for the next six years.

There were clear signs in Lieberman's record that he did not really believe in progressive principles, and they were overlooked to Democrats' regret. Arlen Specter's entire record is a nearly 50-year legacy of opposition to Democratic values that's impossible to excuse.

Arlen admitted that the only reason he made the "painful decision" to leave the GOP was that the polls showed he couldn't beat his Republican challenger. Democrats should have higher standards than a jilted Republican on the rebound.

Specter also boasted that he would not be a reliable supporter of President Obama. When asked on what issues he disagreed with the President, he joked that he didn't have enough time to go through them all. It's a joke Pennsylvania Democrats should take very seriously.

Since I entered the race, Arlen has systematically flip-flopped on positions he staked out even after switching parties. Remember that before I stated my intention to challenge Arlen, he was dead set against any health care public option. He has since reversed course on the Employee Free Choice Act, Don't Ask Don't Tell, and the Defense of Marriage Act, with surely more to come. He may be voicing support for Democratic principles during the campaign, but we cannot afford to ignore the "Lieberman Lesson": It is what Senators do in the six years after elections, not what they say in the run up to them, that counts.

If President Obama is to be successful, if we're serious about fixing this country, then we need Democrats in the Senate who actually believe in what the President is trying to accomplish. After all, the biggest problem for the President isn't Republicans or tea parties, it's a lack of accountable leadership. Democrats have enough votes to pass legislation through both the House and Senate, but there are Democratic politicians who are standing in the way of the right bill.

The name of the game in Congress is how to get President Obama's agenda past the Joe Lieberman's of the Senate. The question for Pennsylvania is, do we need another one?

We should remember the old saying: Fool me once, shame on you; fool me twice, shame on me.

Cross-posted from Huffington Post