HARRISBURG — A national political organization that helps Democrats get reelected to the U.S. Senate is spending money to help Arlen Specter fend off a primary challenge in Pennsylvania.

A spokesman said today that the Democratic Senatorial Campaign Committee is helping pay for a Specter ad on TV that shows him speaking against the backdrop of Philadelphia's skyline.

Spokesman Eric Schultz would not say how much the organization spent on it, or whether it plans further aid. Specter campaign manager Chris Nicholas would not comment.

The ad comes as a new Quinnipiac University poll shows challenger Joe Sestak closing on Specter, who switched parties last year.

The poll says Specter still leads, 47 to 39 percent, but that's narrowed from a 21-point gap a month ago.