WASHINGTON -- President Barack Obama tried to keep Joe Sestak out of the Pennsylvania Senate race, then campaigned for his primary foe. But now that the Delaware County congressman is the Democratic Party's nominee, the president has Mr. Sestak's back.

The Sestak campaign announced Friday that Mr. Obama will headline a fundraising event for Mr. Sestak on Sept. 20 in Philadelphia.

In addition, the president will participate in a fundraiser for the Democratic National Committee, also in Philadelphia, and a third event "on the important issues facing America's families," according to a White House official. Further details were not released Friday.

"We're honored that the president would do an event with Joe during his trip to Pennsylvania," Mr. Sestak's spokesman Jonathon Dworkin wrote in an e-mail. "He and Joe may have had their differences, but they can agree that it is critical to elect a senator who will put Pennsylvania's working families first."

The differences have been more on a personal than a policy level. Mr. Sestak has steadfastly backed Mr. Obama's agenda, from the stimulus bill to health care and financial regulatory reform. But Mr. Obama enticed Sen. Arlen Specter to switch parties last year with the promise of support -- and he provided it, appearing at a Philadelphia fundraiser and dispatching Vice President Joe Biden to Pittsburgh to rake in contributions for Mr. Specter.

Mr. Obama also tried to get Mr. Sestak out of the race, with presidential chief of staff Rahm Emanuel enlisting former President Bill Clinton in the effort, according to a version of events provided by the White House and Mr. Sestak.

Mr. Clinton offered the former Navy admiral an unpaid advisory role on national security matters, but Mr. Sestak nipped the discussions in the bud. He then took on the state and national Democratic apparatus to defeat Mr. Specter convincingly in May's primary election.

The party then rushed to embrace its new nominee for the fight against Republican Pat Toomey. Mr. Sestak insisted there were no hard feelings and maintained he would be honored by a presidential visit on his behalf, though he said he asked the White House to send First Lady Michelle Obama -- who's more popular than her husband. Yet the reason he wanted the first lady, Mr. Sestak said, is because "it's a woman's world."

"It's been an awkward relationship between the president and Congressman Sestak, over this last year at least," said Christopher Borick, a political science professor at Allentown's Muhlenberg College.

"So this is an interesting pairing, and one of, I would argue, mutual necessity. ... Putting all the baggage of the past behind them is in both their interests."

As Mr. Borick pointed out, Mr. Sestak could use the cash: Mr. Toomey holds a significant financial advantage, allowing him to air statewide television advertisements all summer while Mr. Sestak went on the air for the first time this week. Mr. Toomey's advantage has been bolstered by outside groups, such as the U.S. Chamber of Commerce and the Club for Growth (a pro-business group Mr. Toomey used to lead), which have run negative ads against Mr. Sestak.

Mr. Obama also has a stake in keeping the Senate Democratic next year -- a once-sure proposition that the sinking economy has put in jeopardy.

The Toomey campaign took the president's appearance as an opportunity to emphasize Mr. Sestak's support for his policies.

"The president is always welcome in Pennsylvania," Toomey campaign spokeswoman Nachama Soloveichik said in an e-mail. "He's especially welcome this time of year, so he can remind Pennsylvanians that Congressman Joe Sestak voted for all the bailouts, the record national debt, the cap and trade energy tax and the Washington health care takeover."

The visit also is likely to reignite talk of the "job offer" controversy, as congressional Republicans have called for a full-scale investigation into whether the Clinton-Sestak discussion constituted an illegal bribe.

But any unfavorable associations are outweighed for the Sestak campaign by the financial windfall from Mr. Obama's presence, Mr. Borick said. Last September's Specter-Obama event raised more than $2 million, split between Mr. Specter's campaign and the Democratic Senatorial Campaign Committee.

The Sept. 20 event will not split proceeds with the Campaign Committee, according to a committee spokesman.

Also, Mr. Obama could help provide a boost at a time when all the energy nationwide seems to be on the Republican side.

"All signs point to Democrats being in a malaise, and therefore having a president come to visit -- and one that did exceptionally well in southeastern Pennsylvania two years ago -- is one of the ways to try and spark a little bit of energy among the rank and file and especially the base," Mr. Borick said.

"There's no way that Joe Sestak wins in November without getting decent turnout in southeast Pennsylvania among Democrats."

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