WASHINGTON -- Sen. Rick Santorum, a self-described compassionate conservative intent on climbing the Republican leadership ladder, filters all politics and policy through one guiding principle: What is best for the American family.

Two-parent families, says Santorum, are good. Requiring people to work is good. So is banning late-term abortions and giving religion a greater role in government. Traditional welfare, on the other hand, hurts the family. Homosexuality, feminism, liberalism all undermine the family. Even parts of the U.S. Constitution can harm the family.

"If the Supreme Court says that you have the right to consensual (gay) sex within your home, then you have the right to bigamy, you have the right to polygamy, you have the right to incest, you have the right to adultery. You have the right to anything," the Pennsylvania lawmaker said in a recent interview, fuming over a landmark gay rights case before the high court that pits a Texas sodomy law against equality and privacy rights.

"All of those things are antithetical to a healthy, stable, traditional family," Santorum said. "And that's sort of where we are in today's world, unfortunately. It all comes from, I would argue, this right to privacy that doesn't exist, in my opinion, in the United States Constitution."

It's this kind of strong ideology, plus ambition, that has propelled Santorum, 44, through the ranks of the Senate Republican leadership at what his GOP colleagues describe as a meteoric pace. After fewer than 10 years in the Senate, Santorum is No. 3 in the GOP leadership, serving as the party's conference chairman. Should Senate Majority Leader Bill Frist, R-Tenn., step down in 2006, as he has vowed, Santorum will likely seek the post.

In the near term, Santorum is expected to play a leading role in trying to win Pennsylvania -- the nation's fifth-largest electoral prize -- for President Bush in 2004. Bush lost the state in 2000 by 200,000 votes.

Santorum arrived in the Senate in 1995 after four years in the House, and he immediately got a reputation as a brash, confrontational lawmaker who had little respect for seniority or decorum. He has since smoothed the rough edges and often crosses the political aisle to work with Democratic colleagues. At the same time, the style of the Senate has become more sharp-elbowed, with more than half its members, like Santorum, products of the House.

"He tramped on a few toes in those early years - by the way, they were toes, I think, he should have tramped on," said Sen. Orrin Hatch, R-Utah, who was elected to the Senate when Santorum was in his first year at Penn State University. "But he has gradually gotten to the point where he is respected by both sides for his energy, his effort, his intelligence and his decency."

Though he has toned down his impulsive bravado, Santorum remains "every bit as conservative as he ever was," said Rutgers University congressional scholar Ross Baker.

"I would say this is the most conservative Republican caucus in the history of the U.S. Senate, and they've kind of moved in his direction," Baker said. "He has retrofit his ideology with a smooth veneer. You see this sort of transformation taking place in people who have aspirations."

The Catholic son of an Italian immigrant, Santorum, a Pittsburgh lawyer and state legislative aide, defeated seven-term Democratic Rep. Doug Walgren to win election to the House in 1990. He served four years, taking a lead role on welfare reform in 1993, before challenging and defeating Democratic Sen. Harris Wofford in 1994, the year of the Republican revolution.

"In 1994, he fit the boilerplate of the angry young white man in the year of the angry young white man," said Democratic consultant Paul Begala, who worked on the Wofford campaign. "I think since then, he has tried to grow into the job."

In the Senate, Santorum has focused largely on social issues that are the core of Bush's "compassionate conservative" campaign platform. He successfully pushed legislation banning late-term abortions as well as a bill to give tax breaks for donations to religious-based charities. He has hired welfare recipients in his district offices, where he also staffs community affairs liaisons specifically tasked to help faith-based organizations get funding.

Santorum said welfare reform "was sort of my baptism by fire" in shouldering social issues. The law, which must be reauthorized this year, required welfare recipients to work to collect benefits.

"The more I got involved, it really did open my eyes to how the left had destroyed so much," Santorum said.

White House political director Ken Mehlman calls Santorum "one of the original compassionate conservatives."

"The people of Pennsylvania, no matter who they are or where they're from, understand how hard he's working for them, understand that he has a philosophy that is good for them, and that will help improve their state," Mehlman said. "There is nobody who works as hard as he does in getting his message out."

But critics are counting on Santorum's message to hurt him instead of help.

"People are rational about most things, and when you listen to most of what comes out of Rick Santorum's mouth -- he's lost some sense of rationality," said Neil Oxman, president of Philadelphia-based The Campaign Group, who worked on the Walgren campaign in 1990. "It's just kookiness. It's a white-bread world where most people don't live, and it's certainly not Pennsylvania."

Santorum is every bit the family man at home that he is in the Senate.

He and his wife, Karen, have seven children -- including, as Santorum puts it, "the one in Heaven." Their fourth baby, Gabriel Michael, died in 1996, two hours after an emergency delivery in Karen Santorum's 20th week of pregnancy. The couple took Gabriel's body home to let their three other young children see and hold the baby before burying him, according to Karen Santorum's book of the ordeal, "Letters to Gabriel."