Sen. Arlen Specter has a commanding lead over rival Rep. Joe Sestak in the Democratic primary with just six weeks to go in the campaign, according to a new Quinnipiac University poll of likely primary voters.

Specter leads 53 percent to 32 percent among Democrats who say they are sure to go to the polls for the May 18 primary, the poll finds. Just 15 percent of respondents say they are undecided at this point, not enough to swing the race.

"Not only would Sestak have to win every undecided vote, he also would have to take away some who say they are for Specter," said Peter A. Brown, assistant director of the Quinnipiac University Polling Institute.

"While that is certainly possible, it is a steep hill for Sestak to climb because Specter is such a known quantity to Pennsylvania Democrats, who generally like him," Brown said.

Likely Democratic primary voters view Specter favorably by a 60 to 26 percent margin, while they see Sestak favorably by a 33 to 6 percent margin, with 58 percent saying they don't know enough about him to form an opinion.

Sestak has not yet run television advertisements, however, and it is unclear from the poll whether Specter's support is soft. A five-term senator, Specter was a Republican until last year.

Quinnipiac pollsters interviewed 921 likely Democratic voters by telephone between March 31 and April 5 by telephone. The sample was taken from lists of registered voters with a record of voting in recent elections, a method considered more reliable than asking people to describe their voting histories.

Results are subject to a margin of error of a little more than 3 percent, the poll said.

Contact staff writer Thomas Fitzgerald at 215-854-2718 or tfitzgerald@phillynews.com.