FOLSOM, Pa. — Rep. Joe Sestak this morning said he would challenge Republican-turned-Democrat Sen. Arlen Specter, setting the stage for a Pennsylvania primary race likely to center on Specter's Democratic credentials.

Sestak, a former Navy vice admiral, made the announcement at a VFW hall in Folsom in his suburban Philadelphia district.

The second-term Democrat said he wants to put "principles over politics."

"The great American dream is not about getting ahead," he said. "It is about doing well ... and creating a world for the next generation in which they are inspired to do the same."

In April, Specter severed his decades-long ties with the Republican Party. He said, in part, it was to avoid a Republican primary challenge from former Rep. Pat Toomey, who nearly beat him in the 2004 primary.

Sestak, 57, faces an uphill battle in challenging the White House-backed Specter. Sestak has $4.3 million in cash, while Specter has $7.6 million.

Much of Sestak's organizational and financial support could likely come from those in the liberal wing of the Democratic Party who have been slow to embrace Specter.

Sestak graduated from the U.S. Naval Academy and commanded an aircraft carrier battle group during post 9/11-operations in Afghanistan. He has graduate degrees from Harvard University and was director for Defense Policy on the National Security Council during the Clinton administration.

After more than 30 years in the Navy, he returned to Pennsylvania to run for office. In 2006, he defeated GOP Rep. Curt Weldon. He won re-election in 2008.