Dan Pfeiffer, deputy communications director for the Obama campaign, confirmed that Casey would announce his support during a rally at the Soldiers and Sailors Military Museum and Memorial and that he would then set out with the Illinois senator on part of a six-day bus trip across the state.

The endorsement comes as something of a surprise. Casey, a deliberative and cautious politician, had been adamant about remaining neutral until after the April 22 primary. He had said he wanted to help unify the party after the intensifying fight between Obama and Sen. Hillary Rodham Clinton.

"There are few stronger advocates for working families in Pennsylvania than Sen. Casey," Pfeiffer said.

By coming out for Obama, Casey puts himself at odds with many top state Democrats - including Gov. Rendell, Rep. John P. Murtha and Mayor Nutter - who are campaigning for Clinton.

The endorsement also comes at a crucial time for Obama, who has been trailing Clinton in Pennsylvania polls by double-digit margins but who also has bought at least $1.6 million worth of television advertising statewide in the last week, more than double Clinton's expenditure.

Obama strategists hope that Casey can help their candidate make inroads with the white working-class men who are often referred to as "Casey Democrats." This group identifies with the brand of politics Casey and his late father, a former governor, practiced - liberal on economic issues but supportive of gun rights and opposed to abortion. (Obama favors some gun-control measures and backs abortion rights.)

Obama badly lost the white working-class vote to Clinton in Ohio and Texas on March 4, keeping the outcome of the fight in doubt amid questions about whether he could appeal to a group of voters that has often strayed from the party in presidential elections.

Since then, Obama has been stressing economic issues important to the middle class more often than his calls to reform politics. His campaign's recent TV ads in Pennsylvania also feature blue-collar imagery.

Other state Democrats who support Obama include Reps. Patrick Murphy and Chaka Fattah, and former Lt. Gov. Mark Singel.

Casey sees Obama as an "underdog" in the campaign who sacrificed at the beginning of his career to be a community organizer "in the shadows of the closed steel mills in Chicago," said a source close to Casey who is familiar with the endorsement decision but was not authorized to speak publicly about it.

The source, reached by The Inquirer yesterday, said that Casey was also impressed with how Obama had stood up to the pressures of the campaign, including recent attacks over the racially incendiary remarks of his former pastor.

Casey's decision was also personal, motivated in part by the enthusiasm his four daughters - Elyse, Caroline, Julia and Marena - have expressed for Obama, the source said. "He thinks we shouldn't be deaf to the voices of the next generation."