As we discuss his new book, The Gift of Rest, in his Capitol Hill office, Sen. Joe Lieberman mentions his longtime friendship with National Review’s late founder, William F. Buckley Jr. Over 20 years ago, in 1988, Buckley famously endorsed Lieberman, who was challenging moderate Republican incumbent Lowell Weicker. The pair, both with ties to Stamford, Conn. — Lieberman a native, Buckley a resident — remained close following Lieberman’s surprise victory. They exchanged letters and phone calls, and mused about politics and family.

“He teased me that year, to please not indulge in the fantasy that he would ever again vote for me,” Lieberman chuckles. Nonetheless, Buckley, he says, became a “major force in my life.” The political gap between Lieberman, a devout Orthodox Jew and Democrat, and Buckley, a devout Catholic and conservative, may have been steep, but the respect for each other’s intellectual and spiritual pursuits was constant. “It is my view, and I think Bill’s view as well, that faith precedes everything else,” Lieberman says.

Those discussions on faith — with Buckley, with family, with rabbis — have been on Lieberman’s mind this year. Now 69 years old, he has decided not to run for reelection in 2012. Instead of writing a tell-all memoir, or a polemic, he decided to publish a book about his religious life, a love letter to the Sabbath, which he observes, beginning every Friday at sundown, when he turns off the BlackBerry and hangs up the car keys.

In the style of Buckley’s own meditation on faith, Nearer, My God, Lieberman, in The Gift of Rest, encourages readers to mull their own religious practices, placing an emphasis on the value of tradition and commitment. He takes great joy in describing, in vivid detail, the sights and smells of the Jewish Sabbath, from the aroma of baked challah bread and kugel, a sweet noodle dish, to prayers with his children.

Lieberman also acknowledges his struggle, at times, with keeping the Sabbath. Obstacles, he says, have always appeared, from his stressful undergraduate years at Yale to his hectic Senate schedule, which often includes Friday evening or Saturday votes. “I’ve never really talked a lot about it, about why I do it and what we do,” he says. But as his time in Washington winds down, “I thought it’d be wonderful to invite the reader to come to a Sabbath,” he says. “It’s a millennia-old institution, more relevant than ever.”

Lieberman, who in 2000 accepted the Democratic nomination for vice president, is known as a feisty, independent politician. In 2006, he lost his primary battle against liberal Democrat Ned Lamont, only to come back and win the general as an independent. In 2008, he endorsed his friend, Sen. John McCain (R., Ariz.), for the presidency. He assures me that he remains liberal, but when he leaves the Senate, he wants to be remembered for more than his politics.

As the first Jewish politician to land on a major-party ticket, Lieberman, of course, is already in the history books. More important to him, however, is that readers understand that being a leader in Washington — Democrat or Republican — does not mean that a person has to lighten his commitment to his faith or publicly temper his devotion. “Particularly, as I end this chapter in my life,” Lieberman says, he wants to underscore how important it is for “public people, elected officials to speak about faith,” even if such talk is pooh-poohed by politicos.

Lieberman respects Michele Bachmann and Rick Perry, two Republican presidential contenders who have spoken up about their faith on the trail. “I know this got controversial recently, with Governor Perry and Congresswoman Bachmann. But they didn’t give up their First Amendment right to free expression and freedom of religion when they decided to run for president,” he says. “I like it when a candidate, if they feel comfortable, talks about their faith. It’s very interesting to me; it tells me more about the candidate, giving me one more factor to evaluate about what kind of president they would be.”

“Others may be turned off by it, even by the very fact that you’re talking about it, or the way you’re articulating it,” Lieberman says. “That’s the risk you take.” But he emphasizes that while some may find Perry’s public prayers troubling, or Bachmann’s Christian declarations strange, many Americans find such words “reassuring.” In this sense, he urges all politicians, if they are so inclined, to speak up, even if they are not religious experts, in order to make politics more hospitable to religious discussions.

“This is classic America,” Lieberman says. “The Constitution promises freedom of religion, not freedom from religion. The whole history of the country is intertwined with religion. The founding documents are premised on a world view, actually a very creationist world view.” Since then, “We have found a way to invite religion into the public square without pushing all but one religion out. It’s remarkable.”

In the private backrooms of the Capitol, behind the Senate chamber, Lieberman often shares these thoughts with his colleagues, many of whom are Christian. There is a sense, he says, that below the partisan bickering, and all the crosstalk, is a common appreciation of what the United States is, as an idea, and the values that sustain it.

With his new book, Lieberman reflects on these issues, to be sure, but his words, more broadly, serve as a rallying cry for tradition, be it the “gift” of the Sabbath, a tight-knit family, national service, or personal responsibility. It is not a lament, but a reminder that the old ways can bring modern joy, if only embraced. Looking back at his childhood, he remembers: “In those days, those ancient days, we still had the ‘blue laws,’ where a lot of businesses were closed, people didn’t have to go to work, and most of my friends were expected to go to church on Sunday.”

“I grew up in a very Catholic neighborhood, a lot of my friends were Catholic in Stamford,” he says. “When I talk to them about this, they miss it. And I agree. I think the whole society is missing something.” To fill in that void, he says the Jewish Sabbath, regardless of one’s religion, is worth reviewing — especially if it can motivate someone, if only for a moment, to turn off the BlackBerry or unplug the laptop. If a senator can, he laughs, so can you.

— Robert Costa is a political reporter for National Review.