Breaking his avowed neutrality in the presidential nomination battle, Sen. Bob Casey today endorsed his colleague, Sen. Barack Obama.

"This campaign is a chance for America to chart a new course, to go down a different path, a path, first of all, of change; a path of a new kind of politics and finally a path of hope and healing," Mr. Casey told a capacity audience at Soldiers & Sailors Memorial Hall in Oakland.

"I believe in my heart that there's one person who uniquely qualified to lead us in that new direction and that's Barack Obama."

As Mr. Casey spoke, the words from the stage were overwhelmed by chants of "Yes we can; yes we can," from the crowd.

The superdelegate's embrace came as the Democratic presidential contender headed out on a bus tour of Pennsylvania. The rally was to be followed by a stop timed to catch the shift change at US Steel's Edgar Thomson works in Braddock, and further stops in Greensburg and Johnstown as the senator travels east across the state. Polls suggest he trails his rival, Sen. Hillary Rodham Clinton, by a substantial margin in the state.

Mrs. Clinton had appeared in the same hall two weeks ago, showcasing her own Pennsylvania endorsements, including Gov. Ed Rendell, Allegheny County Chief Executive Dan Onorato and Pittsburgh Mayor Luke Ravenstahl.

The decision by Mr. Casey, the top vote-getter in Pennsylvania history, was unexpected as he had said repeatedly that he planned to remain neutral at least until after the end of the state's April 22 primary. Mr. Casey had said it was important for some Democratic elders to remain neutral to put the frayed edges of the party together after the bruising nomination battle.

Mr. Casey said he had notified Mrs. Clinton of the planned endorsement the previous evening.

"She was very gracious. She is a great senator and a great leader," Mr. Casey said.

Mrs. Clinton, who has shown consistent strength in the early polling in Pennsylvania, has the support of most of the state's Democratic hierarchy, including Mr. Rendell, state party Chairman T.J. Rooney, and Philadelphia Mayor Michael Nutter.

Previously, Mr. Obama's highest profile allies in the state were two Philadelphia area congressmen, Reps. Chaka Fattah and Patrick Murphy.

Mr. Casey is broadly popular in the state, and has shown particular strength with some of the voters that Mrs. Clinton is counting on in the state, white ethnic constituencies that fueled her victory in neighboring Ohio.

Beyond whatever persuasive power Mr. Casey might hold for his own constituents, his decision will be closely watched by other superdelegates who will be needed to form a convention majority for either surviving Democrat in Denver. At Soldiers & Sailors today, Casey said his decision would be applauded by yet another audience -- his four daughters, all of whom were Obama supporters before their dad joined them.