Bob Casey Jr. has boosted his fellow U.S. Senator Barack Obama's hopes of closing the gap in the battle to win a majority of Pennsylvania's delegates to the summer's Democratic National Convention.

The surprise endorsement by the state's junior senator will help introduce Obama to those categories of voters who've been least enthusiastic about his candidacy, in particular that of the white working-class.

But Senator Hillary Clinton, who believes she can maintain her double-digit lead in Pennsylvania through polling day on April 22, has the state's chief executive, Gov. Ed Rendell, in her corner.

The governor was elected with the sort of coalition that has propelled the Illinois senator to more than 20 primary victories – white liberals, the young and African Americans.

Pundits feel that the Irish-American Casey is helping to level the playing field. It's thought that his backing will also encourage older voters and Catholics to take a second look at Obama.

"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," he said last week.

Clinton maintains an edge in a state that is similar in its demographics to Ohio, which she won decisively last month.

And the New York senator has deep political roots in the state. Rendell said: "During the '90s, Bill and Hillary Clinton did more for America and more for Americans and more for Pennsylvania and more for Pennsylvanians than any president and first lady in my lifetime."

Hillary Clinton has also called upon out-of-state supporters to make the pitch for her, notably among them some members of an Irish-American family that haven't defected to the Obama side. Robert Kennedy Jr. and his sister Kathleen Kennedy Townsend, the former lieutenant governor of Maryland, wrote a letter to the state's Catholics detailing her policies on health care, foreclosures and fuel bills. (In contrast, their mother Ethel Kennedy introduced Obama as "the next president of the United States" at a ceremony two years ago, and was perhaps the first person to have used the phrase in connection with the Illinois senator.)

Clinton can also rely on Pennsylvania's best-known member of the House, Rep. Jack Murtha. The long-time congressman announced his support for her bid to be president on March 19.

However, one of his protégés, and a fellow centrist Democrat and war critic, 34-year-old Congressman Patrick Murphy threw in his lot with the Obama camp last August.

Murphy, who served in Iraq as a captain with the 82nd Airborne Div., narrowly defeated GOP incumbent Mike Fitzpatrick for a suburban Philadelphia seat that the Republicans had held since 1992. His district, which covers all of Bucks County to the northeast of the city, tends to go with the Democratic Party in presidential elections.

So does the Philadelphia district of another freshman, Rep. Joe Sestak. But the retired three-star admiral, who had strong Irish-American support during his 2006 campaign, has long been behind the Clinton campaign.

Two Irish-American congressmen, however, are sitting on the fence. Chris Carney, another first-term Democrat, is waiting to see which way his northwestern Pennsylvania district goes before saying how he'll cast his superdelegate vote. His district voted 60 percent for George W. Bush last time out and he's said to be one of several Pennsylvania congressmen who are worried about the "coattails" factor – specifically that a Clinton candidacy might mobilize the Republican vote and jeopardize the seats of vulnerable Democratic congressmen.

Such candidates are having a hard time assessing which course would be best for their hopes of retaining their seats. Carney said Clinton's negatives are "widely known," but added, according to Bloomberg News, "What we don't know is whether the other shoe is going to drop for Mr. Obama.”

Carney told media outlets that he has received one call from Clinton and three calls from Obama requesting his support. The Illinois senator has even recruited Carney's Iowa-based mother to make the pitch. "She said: 'Chris, this is your mother talking to you now,'" the congressman reported.

Meanwhile, Rep. Bob Brady, who is chairman of the Philadelphia party organization, is expected to remain neutral until after primary day.