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Mr. Hagel’s visit came on the heels of a recent endorsements from former President Bill Clinton and New York Mayor Michael R. Bloomberg, a three-term incumbent who left the Republican Party to become an independent.

The endorsements underscore the importance of independent voters in the race because both men running for the seat are seen as representing the ultraliberal or ultraconservative wings of their parties.

“This is ideological trench warfare here,” said G. Terry MaDonna, a professor of public affairs at Franklin & Marshall College. “Toomey can’t find anything in the Obama program he likes and Sestak can’t find anything he dislikes.”