Barack Obama saw a critical ally join his ranks today as Senator Bob Casey, champion of the Pennsylvania blue-collar country, delivered his endorsement. Casey’s endorsement spoke to the vigor, unity and vision that Obama uniquely lends to the Democratic Party and to America on the whole:

Appearing on stage beside the Illinois senator, Casey told a boisterous rally, “I believe in my heart that there is one person who’s uniquely qualified to lead us in that new direction and that is Barack Obama.”

“I really believe that in a time of danger around the world and in division here at home, Barack Obama can lead us, he can heal us, he can help rebuild America,” he said.

Most important at this juncture - three weeks and change from Pennsylvania’s vote and on the brink of Obama’s bus tour - is what Casey can help Obama’s campaign with. Unlike Mayor Nutter and Governor Rendell, Casey is a real “swing vote” Democrat - the kind Obama needs to chisel away at the demographics traditionally disposed to Clinton. The key strategic element that wins these kinds of close political battles is that cross-demographic strength, and that’s what Casey can lend: An inroad for the Obama message into the psuedo-Conservative mill worker stock of central Pennsylvania.

Casey is anti-abortion in many regards, and pro-gun in other regards. He is of the same Catholic persuasion as many Pennsylvanian working-class Democrats. He is, in essence, an embodiment of the PA voter Obama needs to haul over to his side. If he wages a muscular, demanding campaign trail for Obama, we could see some serious demolition of the divide separating the two candidates. By contrast, there is not a whole lot that Mayor Nutter, an African-American, can do to sway the urban vote of Philadelphia away from a candidate who has been blooded for the sake of his race in many ways this past week, and who was staunch in not selling them out.

Nutter might damage the Barack base in Pennsylvania’s urban center. Rendell can wield some serious influence. But in Casey, the model of the coarse-cloth politician favored by Pennsylvania’s interior, Obama has gained an ally who can turn the tide of the whole battle.