WASHINGTON -- It was a moment Mary Beliveau had hoped for, and one that Jeanne K.C. Clark had feared.

Sen. Bob Casey last week broke ranks with his fellow Democrats and voted against a bill that would significantly loosen restrictions on federal funding of embryonic stem cell research, a complex and controversial issue that has drawn on some of the longtime angst of the abortion debate.

Pennsylvania's junior senator, Bob Casey, has sided with the Democrats more than 95 percent of the time.

The final vote, held Wednesday night, was 63-34 in favor of the bill. Pennsylvania's freshman senator was one of two Democrats to line up against it.

"We applaud Sen. Casey for standing up for life, sound science, the protection of our tax dollars, and for not caving in to political pressure," said Ms. Beliveau, legislative director of the Pennsylvania Pro-Life Federation.

"I knew there would be some things he'd be OK on," said Ms. Clark, a Democratic activist who heads Women Leaders Online and lives in Shadyside, "and some things he'd be really awful on."

In fact, the stem cell vote was Mr. Casey's only major departure from the party during his first 100 days in office. So far, he's sided with Democrats more than 95 percent of the time, according to a Congressional Quarterly study.

That figure includes votes on increasing the minimum wage, calling for an expansion of a popular health care program for children, and, most importantly for rank-and-file Democrats, setting a deadline to start withdrawing American troops from Iraq.

Mr. Casey, a former state treasurer and son of late Gov. Bob Casey Sr., is willing to go his own way on high-profile social issues, in keeping with a promise he made on the campaign trail last year. But his reliability on other bread-and-butter Democratic issues should keep him in good standing with party leaders.

"Sen. Casey understands better than anyone else how to vote for his constituents in Pennsylvania," said Jim Manley, a spokesman for Senate Majority Leader Harry Reid, D-Nev. "Sen. Reid has a lot of respect for Sen. Casey's deeply held values."

Those values include a strong desire to see a major shift in the nation's Iraq policy.

"The threshold question that every member of the Senate has to answer for themselves is a very basic question. Are you for change? Or are you for 'stay the course'?" Mr. Casey said Thursday at a news conference with other freshmen.

Although some Pennsylvania Democrats objected to his views on abortion and stem cell research, party leaders heavily supported Mr. Casey's candidacy, sensing that his moderate stance would help them defeat Republican Rick Santorum, one of the Senate's most outspoken conservatives.

Iraq, not abortion, took a dominant place in the campaign. Mr. Casey consistently attacked Mr. Santorum's backing of the Bush administration, and he coasted to an 18-point victory, delivering one of the largest defeats for an incumbent in recent political history.

Democrats picked up five other Senate seats, winning a one-seat majority.

"There was a clear and unambiguous call for change in November," Mr. Casey said.

In its first three months, the 110th Congress focused heavily on Iraq. The House moved quickly, passing a resolution that criticized President Bush's proposed "surge" of more than 20,000 extra troops. It also passed a $124 billion emergency spending bill for the war that included a strict withdrawal deadline.

Republicans initially blocked a similar debate in the Senate. They argued that Congress should give the administration's plan a chance to work and warned that lawmakers were sending the wrong message to America's enemies.

At the end of March, however, the GOP allowed the Senate to vote on a version of the emergency spending bill that included a nonbinding timeline to pull out American troops within one year. It passed in a 51-47 vote.

Mr. Casey was an original sponsor of the timeline, which calls for a small number of U.S. forces to stay in Iraq and focus on a narrower mission of counterterrorism and training for Iraqi soldiers. The Pennsylvanian also sits on the Senate Foreign Relations Committee, where he's had a chance to question dozens of witnesses on Iraq policy.

"He's challenged the president with every vote," said Jon Soltz, chairman of VoteVets, an organization that supports an end to the war and backs like-minded politicians. "We brought Iraq veterans with us to lobby the U.S. Senate and he personally met with us. He was one of the few senators to do that. It's more than just a campaign promise."

Yet, because of the natural pace of the legislative process, some of Mr. Casey's campaign promises have been harder to fulfill. Unlike the House, the Senate is a legislative body where all 100 members can have significant power over the day-to-day agenda. At least 60 votes are often needed to win passage for major bills.

Both chambers of Congress have voted to boost the federal minimum wage from $5.15 an hour to $7.25, the first increase in a decade and a top priority for Mr. Casey. In the Senate, Republicans managed to attach tax cuts for small businesses. The House made changes to the bill, but there are still differences, meaning it isn't ready for the president's signature.

Democrats are now trying to include the wage increase with the Iraq spending bill.

"It's maddening that we can't get something so basic and simple as an increase in the minimum wage after a decade," Mr. Casey said in an interview, describing the Bush administration as having a "maniacal" focus on tax cuts for the wealthy.

He noted that Pennsylvania has enacted its own wage increase. On July 1, it will go up to $7.15 an hour.

Mr. Casey was also stymied in his efforts to send more Homeland Security grant money to larger states with more potential targets for terrorists. Not surprisingly, smaller states opposed the plan. And, in the Senate, North and South Dakota have just as much representation as Pennsylvania and California.

Some of his legislative victories include a budget amendment that establishes a reserve fund for pre-kindergarten education and another amendment that calls for an expansion of the State Children's Health Insurance Program, which provides health insurance for almost 6 million children nationwide.

Congressional Democratic leaders also hope to win more federal funding for embryonic stem cell research, arguing that it could develop treatments for a range of medical conditions. But some anti-abortion lawmakers, including Mr. Casey, express concerns that the research involves the destruction of human embryos.

It's a concern shared by President Bush. He vetoed a stem cell bill last year, and he's promised to block the latest version. Both he and Mr. Casey support alternative types of research that focus on stem cells taken from adults or the amniotic fluid of pregnant women.

In the weeks leading up to the most recent vote, Mr. Casey faced lobbying from both sides.

"It's like any important vote," he said. "Part of my job as senator is to listen."

His job also requires him to take firm positions on difficult issues, and he said last week that he wouldn't budge on the stem cell bill, even if his vote could help override a presidential veto.

"In the short run, it will probably lose him a few points with some of the people who supported him," said Thomas Baldino, a political science professor at Wilkes College. "To his credit, Casey stood by his principles."

That's impressed Chris Lilik, chairman of the Young Conservatives of Pennsylvania, although he's hardly ready to support Mr. Casey in 2012.

"I'm pleasantly surprised that he's voting this way," he said. "But, that aside, I can't think of any economic issues that I agree with him on."