SLIPPERY ROCK, Pa. -- Treasurer Bob Casey sparred with his two lesser known rivals, Chuck Pennacchio and Alan Sandals, last night as the candidates for the Democratic nomination for the U.S. Senate debated for the first time.

The trio are vying for the opportunity to take on Sen. Rick Santorum in what is already one of the most closely watched Senate races in the nation.

Mr. Casey's Democratic opponents, Mr. Pennacchio, a history professor, and Mr. Sandals, a lawyer, have throughout this campaign argued that despite Mr. Casey's huge leads in name recognition, money, and support from party leaders, his more conservative stands on social issues such as abortion and gun control would handicap him in the effort to unseat the incumbent.

Each contended that despite their current relative obscurity, their more traditionally liberal positions would make them a more attractive alternative to the Republican in November.

Throughout the 90-minute forum at Slippery Rock University, Mr. Casey took every opportunity to turn questions against the incumbent, portraying Mr. Santorum not as a traditional conservative, but as an ideologue beyond the mainstream of American politics.

The divide between the front-runner and his opponents was clear as they dealt with the question of whether Roe v. Wade should be upheld.

Mr. Sandals and Mr. Pennacchio said that precedent and the right to choose is established and should be protected. Mr. Casey said he opposed the landmark decision, acknowledging that his position would not be universally popular with this audience. But, again, he attempted to use the issue to differentiate himself from Mr. Santorum, noting, for example, that he supported family planning services.

The issue was expected to be a point of difference between Mr. Casey and the other two, but Mr. Pennacchio also used it to attack Mr. Sandals. He accused his fellow Philadelphian of changing his position on the issue of parental notification in order to win the endorsement of the National Organization for Women.

Mr. Sandals dismissed the attack. "You've made a lot of wild charges in this campaign," he said.

The generally civil tone of the debate was also interrupted during this exchange as Mr. Casey admonished Mr. Sandals not to "lecture'' him on the implications of his abortion position.

Mr. Casey was also at odds with his opponents on Sen. Russell Feingold's proposal to censure President Bush over the administration's domestic surveillance program. Mr. Casey said he opposed the proposal as an idle gesture. Mr. Pennacchio and Mr. Sandals strongly supported the censure plan.

They agreed the federal government should do more to expand health care.

Mr. Sandals and Mr. Pennacchio both called for a universal, single-payer approach, while Mr. Casey called for a more incremental approach of expanded coverage for children and aid to small business.

While the exchanges were relatively even-tempered throughout the debate -- televised statewide by PCN -- there was some sniping as the audience began to file out of the university auditorium.

Accusing Mr. Casey of vagueness, Mr. Sandals insisted he still didn't know his position on key issues. Mr. Pennacchio said of the front-runner: "I thought it was ironic [Mr. Casey] was attacking Santorum for taking money from pharmaceutical and insurance [interests] when he's taken the same money."

Asked to respond, Mr. Casey said, "I don't think that's accurate." As always, he took the opportunity to turn the focus to the Republicans.

"Santorum is the leading recipient of special interest money ... half of his money was raised by Mr. Bush and Dick Cheney, rewarding him for his 98 percent support."

Patroling the press room after the forum, Virginia Davis, Mr. Santorum's press secretary, insisted: "all Bob Casey did was attack Sen. Santorum, distort facts and avoid taking positions."

Last night was the first of two debates scheduled for the Democratic contenders.

Mr. Pennacchio and Mr. Sandals were dressed in standard-issue politicians' dark suits. The normally formal Mr. Casey dressed down in a blazer, khakis and an open-necked blue shirt.

The debate figured to be by far the biggest audience that the two long shots had had an opportunity to face in the campaign so far. The size of the hurdle they face in overtaking Mr. Casey was portrayed this week in a new survey from Quinnipiac University. It depicted the Democratic competition as a runaway.

In the telephone poll of Democratic voters, Mr. Casey had the support of 63 percent of those interviewed. Mr. Sandals had just 5 percent; Mr. Pennacchio, 4 percent; and the balance was undecided.

Looking forward to November, Mr. Casey maintained the double-digit lead over the incumbent that he has registered in almost every poll of the race. Mr. Santorum, however, ran ahead of each of the other two Democrats -- 44 percent to 32 percent over Mr. Sandals, and 45 percent to 32 percent over Mr. Pennacchio.

After opening statements last night, the candidates answered a series of questions from moderator Lou Hancherick, the organizer of a group known as North Hills Democracy for America.