WASHINGTON -- Sen. Bob Casey believes that life begins at conception. He supports alternatives to embryonic stem cell research. And he calls himself a proud, "pro-life Democrat."

Yet Pennsylvania's new junior senator quickly drew the ire of a roomful of anti-abortion activists yesterday when he declined to pledge his support for like-minded 2008 presidential candidates.

"I've supported a lot of pro-life candidates, I've supported a lot of candidates who are not pro-life," said Mr. Casey, prompting a chorus of boos and catcalls. "I don't think you could make a determination, in my judgment, about who the president should be based upon their position on that issue."

Tens of thousands of people, including as many as 110 busloads of Western Pennsylvanians, descended on Washington yesterday for the annual March for Life marking the anniversary of Roe v. Wade, the 1973 Supreme Court decision that legalized abortion nationwide.

After the march, about 100 participants from Pennsylvania packed a public forum for an opportunity to gauge Mr. Casey, the politician who last year easily defeated Republican incumbent Sen. Rick Santorum, one of Congress' most vocal opponents of abortion.

Sen. Arlen Specter, R-Pa., also spoke. Both senators took dozens of questions.

Mr. Casey's anti-abortion position caused a stir among many Democrats when he first launched his candidacy. But party leaders decided that the son of late Gov. Bob Casey fit a moderate political mold and had a stronger chance of defeating Mr. Santorum than any other candidate.

That formula proved to be successful across the country, helping Democrats win control of both houses of Congress for the first time in more than a decade.

"I happen to be a pro-life Democrat, and I'm proud to say that," Mr. Casey told the crowd yesterday. "The right to life, as Gov. Casey used to say over and over again, must mean the right to a decent life."

The elder Casey was a strong opponent of abortion, a position that prevented him from getting a speaking role at the 1992 Democratic national convention. That pushed Joan ne Warren, 60, of Cranberry Township, to leave the party.

"Sen. Casey, your father was a man I admired very, very much," said Ms. Warren, who has been attending the March for Life event for 32 years. "How can you stay in the Democratic Party when they treated your father with such disgust?"

Mr. Casey said the treatment of his father had angered him, but both he and the former governor decided to continue as Democrats, hoping to influence the party from within.

Regarding stem cell research, a topic that has infiltrated the abortion debate in recent years, Mr. Casey said he is against research that destroys embryos, but he sees hope in a new study on the research potential of cells taken from the amniotic fluid of pregnant women.

Mr. Casey differs with Mr. Specter, a cancer survivor who speaks frequently about the potential of developing treatments for serious illnesses through advanced research. The state's senior senator weathered harsh criticism during the public forum.

When Mr. Specter noted that many fertility clinics already discard unused embryos, one woman compared the research with Nazi experiments performed on Jews during World War II.

"I consider it very offensive to analogize the Holocaust to stem cells," said Mr. Specter, who is Jewish.

The senator said he personally opposes abortion, but he thinks the government should leave the decision up to individuals and their families, citing the example of the father of the modern conservative movement, former Sen. Barry Goldwater of Arizona, the 1964 Republican presidential candidate.

"He wanted to keep the government off our backs, out of our pocketbooks and, as he put it, out of the bedroom," Mr. Specter said.

Earlier in the day, Archbishop Donald Wuerl said mass for 20,000 people in the cavernous Verizon Center, presiding over the event for the first time since he left Pittsburgh last year to head the Washington, D.C., archdiocese. "It is very exciting to be a part of such a testament to the gospel of life," he said.

The mass followed a youth rally that was as much a celebration of faith as a protest against abortion.

"It's awesome," said 16-year-old Anthony Pulleo of Beaver Falls, wearing a Pitt Panthers jersey and hat. "There are so many people here. It's unbelievable."

He and his friends swayed to the music of a Canadian group, as the singer chanted, "Blessed be the name of the Lord."