Casey said Obama was best placed to help boost a manufacturing industry in the northeastern state.
"He started this campaign as an underdog, but he knows what it's like to be a fighter"
Senator Bob Casey
"He started this campaign as an underdog, but he knows what it's like to be a fighter," he said.
Casey, who is popular among white working-class voters known as "Casey Democrats", is the son of a former governor known for his opposition to abortion and support of gun rights.
Democratic party angst has mounted as Clinton and Obama have traded accusations about each other's performance and pledges.
Howard Dean, the Democratic national committee chairman, warned both contenders that whoever loses the nomination battle must support the victor.
Senator Patrick Leahy, who supports Obama, said there was no way Clinton could win, and urged her to allow Obama to take on John McCain, the Republican nominee.
"Senator Clinton has every right, but not a very good reason, to remain a candidate for as long as she wants to," Leahy said in a statement.
"She ought to withdraw and she ought to be backing Obama. Obviously that's a decision that only she can make.
"As far as the delegate count and the interests of a Democratic victory in November go, there is not a very good reason for drawing this out."
Clinton defeated Obama among working-class voters in Ohio and Texas on March 4.
|Clinton has been urged by some Democrats to |
end her campaign and support Obama [AFP]
She holds a double-digit lead in recent polls of voters in Pennsylvania, which holds the last major primary.
A win there could bolster her claim of momentum in the race even though she lags behind Obama, both in delegates and the popular vote.
Some Democrats are concerned that a nasty primary race could be damaging to its hopes for the presidency, leaving McCain to focus on the general election as Obama and Clinton continue to slug it out.
Clinton needs a win in Pennsylvania to boost her argument that only she can win big states that Democrats must secure to recapture the White House in November.
Neither Clinton nor Obama can reach the magic number of 2,025 delegates necessary to wrap up the nomination.
The task of selecting a nominee thus falls to superdelegates - senior delegates who are not bound to vote according to primary and caucus results.
"I think there's 800 of them and 450 of them have already said who they're for. I'd like the other 350 to say who they're on between now and the 1st of July so we don't have to take this into the convention," Howard said.