WASHINGTON – A week after President Obama’s 2012 re-election, the conservative Federalist Society gathered 1,500 lawyers in black tie to hear one of their own, Justice Samuel A. Alito Jr., vow to hold the line against an ever-expanding federal government that “towers over people.”

He lambasted environmental regulators who fined an Idaho couple $75,000 a day for filling in part of their property that was declared “wetlands.” He derided Obama administration lawyers for telling a private Lutheran school who it could hire as a religion teacher. And he complained that Obama’s health care law had “gutted” limits on the government’s power by forcing citizens to buy insurance.

The message was clear: Obama and the liberals might control the White House, but Alito and the Supreme Court’s other conservatives would have the final word on the law.

President George W. Bush’s second appointee is probably the least-known of the nine justices. He describes himself as “very boring” and can appear stiff and shy in public.

But Alito has proven to be a rising power inside the court and one of its most significant new justices in at least two decades.

Replacing moderate Justice Sandra Day O’Connor, he shifted the court’s ideological balance to the right, providing the crucial fifth vote to relax campaign funding limits, uphold Christian prayers at public meetings, scale back affirmative action, allow greater abortion regulation and strike down part of the Voting Rights Act.

This month, the court will decide three more direct challenges to Obama’s policies in which Alito’s vote will be key. The justices will rule on whether the president exceeded his authority in making recess appointments and whether his environmental regulators went too far by requiring permits for new power plants to show that they are reducing greenhouse gases.

And in the most far-reaching case, the Supreme Court will decide whether business owners have a religious-freedom right to refuse to pay for contraceptives for female employees, as required under the president’s health care law.

With Alito, the court has become a check on the White House, much like then-Sen. Obama called for in 2006 when he joined an unsuccessful filibuster against Alito’s confirmation. “We need a court that is independent,” Obama said – one that “is going to provide some check on the executive branch.”

A few years later, the two men shared another tense moment in the national spotlight during Obama’s 2010 State of the Union speech. The week before, the court’s 5-4 decision in Citizens United freed corporations and unions to spend unlimited sums on independent campaign ads. When Obama accused the justices of “reversing a century of law,” a frowning Alito was seen shaking his head and mouthing, “Not true.”

Asked about the incident, Alito rolled his eyes and smiled, recalling that as a student debater he once lost points in a tournament for making a face when he disagreed with an opponent’s point. “It’s something I should have learned in high school,” he said in a brief interview.

Alito played a key role in the Citizens United case, which began when a conservative group sought to market a video that mocked Hillary Rodham Clinton as she ran for president in 2008. Federal regulators blocked the effort as an illegal campaign expenditure because the group had been funded by corporations.

During the argument, Alito questioned how such a ban made sense in light of the First Amendment’s protection for the freedom of speech. His probing questions shifted the argument. Does the law also permit, he asked incredulously, banning a book that sharply criticizes a candidate if the publisher is a corporation?

“He’s been a terrific justice,” said Washington attorney Charles J. Cooper, who hired Alito in 1986 as his deputy at the Justice Department. “His stature has steadily grown.”

Harvard Law professor Charles Fried, who served as U.S. solicitor general under President Ronald Reagan, gives Alito mixed grades, saying he has been careful and thoughtful, but also too driven by ideology.

“The quality of his work is excellent. He is not a wise guy. He’s doesn’t demean those who disagree with him. And you don’t get pompous sloganeering from him,” Fried said. “But I’m sorry that on the agenda items, he’s been quite predictable. There’s a real sense of an agenda with this court, and he’s been part of that.”

And where O’Connor said the government may not “endorse” or favor a particular religion, Alito helped form a 5-4 majority in May that said local officials may sponsor explicitly Christian prayers at their public meetings.

The ruling led to a sharp exchange between Alito and Justice Elena Kagan. In a strong dissent, she said the court had wrongly approved a small town’s policy of “religious favoritism,” whereby every month for eight years, a Christian minister offered a prayer to open the council meeting, despite objections from Jewish residents.

Replying to her, Alito said public prayers are as old as the nation. They need not be “perfunctory and hidden away,” he said, calling her dissent “really quite niggling.” That comment, Kagan replied, “says all there is to say about the difference between our respective views.”

Away from the court, friends say Alito, 64, is modest and unpretentious. His wife, Martha-Ann, a former law librarian he married 29 years ago, is as outgoing and sociable as her husband is quiet.

His passion outside of work is baseball. The lifelong Philadelphia Phillies fan’s chambers at the court has a wall devoted to memorabilia.

Alito’s conservatism was forged in his youth. He was raised by Italian Catholic parents in a middle-class suburb of Trenton, N.J. His mother was an elementary school principal, and his father an independent analyst for the State Legislature.

Alito signed up for the Army ROTC in 1970, only to see it shut down on campus in response to antiwar sentiment. When he joined a group of Princeton students on a trip to Washington, his goal was to meet Justice John Marshall Harlan, a conservative who was known as the “great dissenter” on the Warren court.

By the time Alito left college, he was a quiet conservative with a large ambition. He told the Princeton yearbook he planned to go to law school and “eventually to warm a seat on the Supreme Court.”

Though he is sometimes criticized for being pro-business, he has worked his entire career for the federal government: as a law clerk, prosecutor, Justice Department lawyer, U.S. attorney, appellate judge and now Supreme Court justice.

“The confirmation was hard on him,” said a former clerk who asked not to be named. “He felt the attacks were very mean-spirited, and he felt like his family was attacked.”

The Democrats who sharply questioned him included then-Sen. Joe Biden of Delaware.

Alito has denied that the Senate hearings affected his work, but there are signs they are not forgotten.

In January 2009, a newly inaugurated President Obama and Vice President Biden stopped by the Supreme Court for a brief social call. All of the justices were there to receive them – except Alito.