Why Can't Barack Obama Close the Deal?

The candidate who burst onto the national stage promising to bring red and blue states together is suddenly looking quite blue.

Sen. Hillary Clinton, D-N.Y., claimed an overwhelming victory in Puerto Rico Sunday however the candidate faces an uphill battle as Sen. Barack Obama, D-Ill., is inching closer to the nomination.

(ABC News Photo Illustration)

Sen. Barack Obama's second consecutive lopsided loss in a critical swing state has exposed soft spots in the support he's been able to secure.

The Illinois senator's had persistent problems in winning working-class, less-educated whites and Pennsylvania accentuated his seeming inability to connect with those voters.

Key Losses Fuel Doubts

While Obama remains the prohibitive front-runner -- with an effectively insurmountable lead in elected delegates -- those potential weaknesses among key demographic groups are fueling a fierce argument inside the Democratic Party over Obama's ability to win a general election.

Related

The Note

Dems' Bitter Path to the Bitter End

Clinton's Pa. Win Sends Dems to N.C., Ind.

The weaknesses constitute Sen. Hillary Clinton's last best shot at the nomination: convincing party insiders who serve as superdelegates that Clinton can turn out the voters who are critical to the Democrats' chances against Sen. John McCain, R-Ariz.

"The bar was set high in Pennsylvania and she got over it with room to spare," said Steve Grossman, a former Democratic National Committee chairman who is helping the Clinton campaign lobby superdelegates. "She's been able to build a broader coalition in each of the last three primaries. And I think that his [Obama's] coalition is more available to her than hers is to him."

Obama Fails to Deliver Knockout Blow

Despite being badly outspent by Obama, the New York senator won a resounding, nearly 10-point victory in Pennsylvania on Tuesday.

It was a statewide Obama wipeout: He carried only seven of the state's 67 counties, primarily in the Philadelphia and Harrisburg areas in the southeastern portion of the state.

"In the big states that count -- that we must carry to win the Electoral College -- she's the best candidate," Pennsylvania Gov. Ed Rendell, a Democrat, said on a conference call with reporters Wednesday. "It was a victory virtually everywhere in the state. … This was an awesome victory, a landslide."

Clinton started making that argument to superdelegates in the immediate aftermath of the primary.