Bob Casey's lead over U.S. Sen. Rick Santorum has stretched to 18 percentage points since early May, as Mr. Casey picked up new support and the Republican incumbent's approval rating skidded to a four-year low, according to a poll released today.

Mr. Casey, Pennsylvania's Democratic state treasurer, leads Mr. Santorum 52 to 34 percent --- the biggest margin since October, when the numbers were the same, according to the Quinnipiac University poll.

Only 38 percent of the respondents said they approved of the way Mr. Santorum, the third-ranking Republican in the Senate, is handling his job. Forty-five percent said they disapproved and 16 percent did not express an opinion.

It was the first time the senator's approval rating had dropped below 40 percent since Quinnipiac began measuring it in June 2002. It also was the latest sign of distress for the outspoken conservative, an ally of President Bush on issues that included a constitutional amendment banning same-sex marriage and private savings accounts for Social Security.

President Bush's approval rating in the state rebounded somewhat in the latest poll, to 34 percent from 30 percent in May. And the proportion of Pennsylvania voters who approved of Mr. Bush's handling of the war in Iraq increased to 35 percent from 29 percent.

"Sen. Santorum appears to be his own worst enemy in his battle for re-election," said Clay F. Richards, assistant director of the Connecticut-based university's polling institute.

In the May survey, Mr. Casey, the son of the late governor, led Mr. Santorum 49 percent to 36 percent.

The latest survey had Mr. Casey running strongest in his native northeast region, as well as in and around the Democratic strongholds of Philadelphia and Pittsburgh. Mr. Santorum garnered less than 40 percent support in all areas except central Pennsylvania, where 42 percent of voters backed him.

Among Republicans who consider themselves conservatives, three-fourths support Mr. Santorum, compared to barely half of the GOP voters who say they are moderates or liberals.

A spokeswoman for Mr. Santorum said polls are unreliable at this stage of the campaign.

"The candidates haven't debated. Neither of them are on the air with television ads," said the spokeswoman, Virginia Davis. "Polls don't mean anything at this point."

Larry Smar, speaking for the Casey campaign, agreed that it is too early to read too much into the polls but said that Mr. Casey's continuing strength in polling bodes well for his prospects.

Mr. Santorum's drooping approval rating shows "Pennsylvanians have had enough of Rick Santorum and they want a change," Mr. Smar said. "They're ready to change horses."

In the poll, Quinnipiac conducted telephone interviews with 1,076 Pennsylvania voters between June 13 and Monday. The results carry a sampling margin of error of plus or minus 3 percentage points.