This is one in a seven-part series featuring the major GOP presidential candidates.

WASHINGTON — For former Sen. Rick Santorum, it’s always been about faith.

Deep religious faith fuels Santorum’s conservative politics. It’s what propelled him into becoming one of Congress’ leading opponents of abortion, same-sex marriage and wrongdoing by fellow lawmakers, regardless of party affiliation.

Faith is the key ingredient that also powers Santorum’s long-shot drive for the 2012 Republican presidential nomination. Though solidly in the bottom tier among the seven remaining major GOP candidates, the former Pennsylvania senator doggedly soldiers on through the cold of Iowa and New Hampshire and the temperate early winter of South Carolina. He remains confident that his campaign will catch fire among conservative voters who may be leery of the current top-tier favorites — former House Speaker Newt Gingrich and former Massachusetts Gov. Mitt Romney.

“He impresses people because he’s committed to running a campaign that spotlights his deep feelings on issues that he feels are important,” said Tom Rath, a veteran New Hampshire Republican activist and Romney strategist.

Santorum, 53, is a man waiting for his moment at the top of the GOP presidential heap — a position that’s already rotated among Romney, Gingrich, Texas Gov. Rick Perry, Rep. Michele Bachmann of Minnesota and businessman Herman Cain.

“Rick Santorum believes that he can win and believes that he can be the last person standing among the candidates,” said G. Terry Madonna, a public affairs professor at Pennsylvania’s Franklin & Marshall College. “What he aspires to be is not blown out in the early events … and to be the alternative to Romney.”

But Santorum’s moment has been elusive thus far; perhaps, some political analysts believe, because Santorum is so closely identified with social issues in an election season where jobs and the economy are the dominant concerns.

“Everyone has had a turn except Rick, and the million-dollar question everybody is asking — and he’s probably asking — is why,” said Tony Perkins, president of the Family Research Council, a conservative social issues-oriented group. “He’s the one candidate that could stand the scrutiny. He could still have his moment.”

The son of a psychologist father who was an Italian immigrant and a nurse mother, Richard John “Rick” Santorum was born May 10, 1958, in Winchester, Va., but grew up in Butler, Pa., a western Pennsylvania town he’s described as blue-collar.

A devout Catholic and father of seven children, Santorum was elected to the House of Representatives in 1990 at age 32. He was a member of the so-called “Gang of Seven” House GOP freshmen who rankled House leadership in both parties by highlighting check-writing abuses by their fellow lawmakers at the now-defunct House bank.

The House bank scandal — which ensnared several Democrats and a few Republicans — helped lift the career of Gingrich, R-Ga., and helped Republicans take control of the House in 1994 for the first time in 40 years.

It also helped Santorum “make his bones” in Congress, Madonna said. In 1994, Santorum defeated incumbent Democratic Sen. Harris Wofford, and he soon rose to become Senate Republican conference chair, the No. 3 leadership position.

In the Senate, Santorum became known for his social conservatism. He led efforts to ban late-term abortions and led the unsuccessful GOP fight in 2005 to keep Terri Schiavo, a Florida woman who was diagnosed as being in a persistent vegetative state, attached to life-preserving medical equipment.

He derided a 2003 Supreme Court decision that declared a Texas sodomy law unconstitutional, saying, “If the Supreme Court says that you have the right to consensual 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.”

Santorum’s stances earned him a solid following among religious conservatives and a spot on Time magazine’s 25 most influential evangelists list in 2005.

It also earned him the enmity of many Democrats, women’s groups, abortion rights advocates and gay rights supporters, who disliked what they considered Santorum’s holier-than-thou attitude.

“Santorum, he’s got a bit of a brash ‘in your face’ personality, which hasn’t helped him politically at times,” Madonna said. “Having said that, he’s very smart and a very good campaigner. But that style does have the effect of turning off some voters.”

Santorum lost his seat in 2006, suffering a crushing 18-point defeat by current Democratic Sen. Bob Casey, the son of a popular Pennsylvania governor.

It was one of many GOP defeats that year chalked up to the public’s anti-incumbent, anti-war mood. But Santorum also suffered from thorny questions in the state about exactly where he lived.

He listed his legal address as a home in Penn Hills, Pa., a Pittsburgh suburb, but he also had a larger home in Leesburg, Va., a Washington suburb. Five of Santorum’s older children were educated in Virginia via a western Pennsylvania online charter school. Eighty percent of their tuition was paid by the Penn Hills School District. But school district officials in 2004 charged that Santorum’s family didn’t meet residency requirements and sought to get tens of thousands of dollars’ worth of tuition reimbursement payments back.

Santorum said he’s now a Virginia resident.

“We’ve been voting in Virginia the last few elections,” he told reporters in Harrisburg, Pa., in September. “I officially changed my voter registration, I think, a year or so ago.”

Santorum’s 2006 defeat was devastating, but not as much as some moments in his personal life. He and his wife, Karen Garver Santorum, had a son, Michael Gabriel, who was born in 1996 and lived for only two hours.

The couple slept with the lifeless infant in the hospital that night and took him home so the other siblings could hold him before he was buried, Santorum’s wife recounted in her book “Letters to Gabriel.”

And Santorum campaigns as his youngest daughter, 3Ω-year-old Isabella, battles trisomy 18, an often-fatal genetic disorder.

Isabella’s life expectancy isn’t “particularly long, and just the idea of going off and doing something like this was something I really struggled with,” Santorum told The Washington Post last month.

He added: “I feel like I wouldn’t be a good dad if I wasn’t out here fighting for a country that would see the dignity in her and every other child.”