Susan Margaret Collins is more optimistic and habitually cheery than her sometimes-downbeat Senate sister, Olympic Snowe. On a snowy day, she welcomes a constituent, a man in a plaid shirt and jeans, into her office for a photo and sounds like a joyful first-grade teacher leading a field trip.

“Oh my gosh,” Collins exclaims after he hands her a bumper sticker that says Maine-iac, a nickname for native Mainers. “I love it! You know, I have found these all over the world and stuck in the silliest places.”

Her voice has an unusual cadence — a quavering staccato. It seems on the verge of stalling, but never does. When concerned inquiries are made to her staff about whether this stems from a medical condition, the reply is, “No, that’s just how she talks.”

Athletic and trim, wearing a gray and black workhorse suit and low-heeled patent leather pumps, Collins seems younger than her age, 58, and her blue eyes radiate intelligence, good will and a sturdy confidence that can be infectious.

Just a building away from Snowe’s green office with its clutter of watercolors, souvenirs and crystal trophies, Collins dwells in a spare space with flags, state seals and New Englandy furnishings. The walls are midnight blue. The wainscoting is dark mahogany. The carpet is terracotta orange. A dead orchid plant sits on a shelf behind her desk.

The phone rings and it is Mitch McConnell. The minority leader is ready to hand out committee assignments for the 112th Congress. Collins slips into another room to talk to him — “in case I get bad news” — and returns five minutes later, glowing. “I got everything I wanted!” Her committee load is so heavy — Armed Services, Appropriations, Homeland Security and Governmental Affairs — that it requires a special waiver of Senate rules.

Two staffers pop their heads in the door to hear the outcome and congratulate the boss. “More work for you!” she calls out, with a sing-songy laugh.

More work for Collins, too. And what could be better? She is quick to mention her love of cooking, and her cabin on a lake north of Bangor where she kayaks and spends three weeks in summer. But for the 49 other weeks of the year, she is a prodigious grind.

Never married and, like Snowe, without children, Collins is known for carrying a bulging briefcase home in the evening, for night-owl emails to exhausted staffers and for plunging into every bill on her desk and catching mistakes and oversights, such as the 17-word clause she spotted, inserted by an anonymous colleague in the 1999 budget, that would have kicked back $46 billion to the tobacco industry.

“She does 100 things at once,” says Steve Abbott, her former chief of staff, “goes from committee meeting, to constituent meeting, and on to another hearing, without missing a step.” She is a classic junior senator in this way: happiest when she’s trying harder.

“I truly enjoy legislating,” Collins says, pausing for a 30-minute interview, all she has time for today, before an informational meeting with Us Against Alzheimer’s. “I love bringing people together from both sides and sitting down and figuring out what ideas they have. That’s exciting for me. And that’s why I think Maine has sent me here — not to be particularly ideological, although I am proud to be a Republican, but to solve problems and to work with people who are interested in solving problems.”

When Snowe, as The Listener, describes herself as “a problem solver,” it has an emotional component, a poignancy. She would fix life if she could, to soften its blows. Collins’ connection to her work seems strategic and process-loving, as though she is talking about a puzzle she’s passionate about piecing together, or a clock she’s itching to repair.

Flinty pragmatism is a way of life where she comes from, the northern town of Caribou, population 12,000, where Collins family members have run a lumber yard for six generations and have been civic leaders for four. “My family taught me that you had no right to complain about the outcomes,” she says, “if you didn’t care enough to get involved.”

Her parents, Donald and Patricia Collins, both did turns as mayor of Caribou. Her father, as well as her grandfather and great-grandfather, served in the Maine legislature. An uncle sat on the state Supreme Court. This year, when Collins was inducted into the Maine Women’s Hall of Fame on March 19, she was following a trail blazed by her mother — a longtime trustee of the University of Maine — who was inducted in 2005.

A work ethic was instilled when 9-year-old Susan was paid 30 cents for each barrel of potatoes she dug up at a neighbor’s farm. And being a middle child, the third of six, may explain her skill at collaboration.

As a young girl, she was a strong swimmer, a sailor, an honor-society-type student and a neighborhood babysitter who was legendary for her fairy tales and games. “She entertained us,” says her younger brother Sam, who runs S.W. Collins Building Supplies with their youngest brother, Gregg.

“I don’t remember Susan having conflicts with me or our other siblings,” Sam says, “or even the usual parent-daughter conflicts.” Susan taught Sam how to ride a bike, how to read, and at a lakeside camp where the Collins family spent the entire summer, she and her sisters organized the “Summer Fun Club,” a shed where, Sam says, “all the kids gravitated for meetings.”

Collins took her first airplane trip as a senior at Caribou High in 1971. A Senate youth program flew her to Washington, where she was taken to meet Maine’s two senators, Muskie and Smith. “Ed Muskie did what I do a zillion times a day,” she says, gesturing to the window where she does her photo ops. “I chatted briefly with him, had a picture taken, and then he was off to a meeting.

“But Margaret Chase Smith took me into her office and talked to me for nearly two hours,” Collins says. The oft-recounted meeting is still fueling her.

“Even though my family was very encouraging of opportunities, there were a lot of mixed messages for women in that era,” she says. “But I left her office that day thinking that a woman could do anything.”

Collins met Bill Cohen, a 32-year-old lawyer running for Congress, when he walked the entire length of Maine’s 2nd District — 650 miles — staying with different families every night. When he got to Caribou, he stopped in to see her parents. Susan, on break from St. Lawrence College, wound up driving a campaign car and the next year, she sent Cohen a letter, which he has kept all these years, asking if she could be a summer intern in his office. She arrived in the summer of 1974, when Cohen was the first Republican in the House to vote for the impeachment of Richard Nixon. The following year, after graduating magna cum laude, she moved to Washington to join his staff.

“Susan was doing pretty serious stuff, high-level Senate hearings at 26 or 27,” says Bob Tyrer, former chief of staff for Cohen and who still works with him at the Cohen Group. “By the age of 30, she had done lots of responsible things.”

A dozen years later, she returned to Maine and became a much-lauded business commissioner in Gov. McKernan’s cabinet for five years, eventually running for the vacant governor’s seat in 1994. Tension between Collins and Snowe is said to have begun at this time, after Snowe and McKernan backed another candidate in the primary. Collins was hurt — dismayed — even though the other contender had more political experience and a deeper connection to the couple.

“Susan didn’t have any great force behind her,” says Abbott, who met Collins then, “just a belief that she was the right person for the job. There was a field of eight candidates. She was the least likely to win the primary, but she did.”

In the general election, she received only 23 percent of the vote. (“I was trounced,” she likes to say.) Conservatives painted her as a liberal, pointing to her support of gay rights. But mostly, nobody knew who she was.

“I was so uncomfortable just walking up to people as a total stranger and introducing myself,” she says. “In Maine, people were always polite, but they looked at me so blankly.”

Collins was a graceful loser, but failure pushed her forward. “She bounced back,” says Tyrer, who helped run her successful Senate campaign when Cohen chose to leave two years later.

That same poise, persistence and cool-headed humility characterizes her time in the Senate. It can take two or three terms for a senator to achieve the ranking to run a high-profile hearing or investigation. But within a few months, because of strategic moves learned during her years as a Senate staffer, Collins became the first freshman senator to lead the permanent subcommittee on investigations, which had found $26 billion worth of fraud in the Medicare system. She landed on the nightly news and on the front of The Washington Post. She jumped aboard the McCain-Feingold campaign finance reform bill, too, which had been languishing and in need of a third Republican sponsor.

Before her first term was over, the former staffer had the makings of a Senate celebrity — and a Cinderella story — something that wasn’t easy for a certain senior senator who had worked her way more deliberately to the Senate, one elected office after another, and who had waited 12 years just to run for her seat. The wintry chill set in.

Collins plowed on — she always does — partly relentless, partly impervious. “Susan has been underestimated throughout her career,” Abbott says. “She doesn’t rely on charm or charisma. She gets things done by being incredibly insightful. And her work ethic is unlike anything I’ve ever seen. Plus, she’s just lucky. Time and time again.”

Luck or pluck? She was chosen with Sen. Lieberman to lead the crafting of the homeland security legislation after the Sept. 11 attacks, because of their close partnership and get-along-with-everybody style. “We had to take on so many people,” Collins says, “including Donald Rumsfeld, who was trying to sink the bill behind the scenes even though the president was for it.”

When she’s being undermined, she has learned to “undermine the underminer” — by finding allies and generating public support for her side. That is how she dealt with Rumsfeld, she says.

“She hung in there, all the way,” Lieberman says. “She is a real competitor, and tenacious.”