Sen. William “Mo” Cowan of Massachusetts is in his last days in office ahead of a Tuesday special election to replace former senator John Kerry, who now serves as secretary of state. (Sarah L. Voisin/THE WASHINGTON POST)

William “Mo” Cowan (D-Mass.) loves being in the U.S. Senate, but he is completely unwilling to do what it takes to get there or stay there. “I’ve come to love the job, but I still hate the application process,” he said. By which, he means raising money and campaigning.

“I think money has such a pervasive, pernicious influence on governing around here that it’s getting in the way,” he said. “And perhaps it’s not even money; it’s the fear of what may happen to you, who may mount a challenge against you if you’re seen as capitulating or compromising too much or violating the principles of your given base.”

The Senate is packed with people who spend millions of dollars to win and keep their seats every six years. That makes Mo Cowan a rare breed. He’s a six-month Senate seat-warmer with no desire to stick around.

An unknown figure in Washington, Cowan was tapped by Massachusetts Gov. Deval L. Patrick (D) in late January to replace John F. Kerry, who stepped down to serve as secretary of state. Patrick chose his former chief of staff for the role against the wishes of other better-known Bay State politicians who wanted the gig.

But Cowan made clear from the start that he did not want the job permanently. He will leave Capitol Hill in the coming days to make way for Rep. Edward J. Markey (D-Mass.), who won a special election for the seat Tuesday, defeating Republican Gabriel Gomez.

“The absence of a campaign, the absence of a need to raise money does put me in a somewhat advantageous situation,” Cowan said recently.

Cowan is living what he calls “the greatest civics lesson that one could ever imagine,” which requires him as a junior senator to occasionally lead the chamber’s proceedings. But his brief tenure also has exposed him to how difficult modern-day politics is making it for lawmakers to accomplish anything other than their own reelection.

“I actually don’t think our system is broken writ large,” he said. “But it is only as good as the people who are in power to use it or employ it, and they’ve got to want to make it work.”

As Patrick’s chief of staff and a longtime confidant, he handled several sensitive matters, including the selection of Paul Kirk in 2009 to temporarily succeed the late Sen. Edward M. Kennedy (D-Mass.).

Cowan fits the mold of recent picks to serve out the terms of longtime senators. New Jersey Gov. Chris Christie (R) this month selected his former general counsel, Jeffrey Chiesa (R), to fill the seat of the late Sen. Frank R. Lautenberg (D-N.J.) until a special election is held in October. When Sen. Daniel K. Inouye (D-Hawaii) died in December, Hawaii Gov. Neil Abercrombie (D) turned to his trusted lieutenant governor, Brian Schatz (D), who plans to run for the remaining two years of Inouye’s old term. In 2010, then-Gov. Joe Manchin (D-W.Va.) asked his chief of staff, Carte Goodwin, to fill the position of the late Sen. Robert C. Byrd (D-W.Va.). Manchin then ran for and won the seat himself.

Cowan is biding his time in Kerry’s old office suite in the Russell Senate Office Building. The main office is painted yellow and thick drapes adorn the windows, but the walls and bookshelves have been stripped bare of Kerry’s past — and Cowan does not dare settle in. He took the job Jan. 30 and is expected to leave when the Senate returns from its Fourth of July recess. His tenure is so short that he decided to live with his wife’s cousins in Silver Spring rather than rent temporary quarters.

“You got to be able to pack and leave in a hurry when you’re an interim senator,” he said.

Six months in the Senate is not nearly enough time to make a significant mark and Cowan has mostly served as a reliable Democratic vote. He is expected to support the bipartisan immigration bill when it earns a final vote this week, and he voted for a far-reaching farm bill this month. A staffer answering phone calls in his office lobby one day recently had to explain to a caller that Cowan supports toughening the military’s standards on sexual assault but is not sponsoring any legislation to do so “because it’s not clear he will be here when the bill is brought up.”

Much of his time has been spent presiding over the Senate chamber and mastering its complex procedural rules. Cowan has cherished the role, which is usually considered a thankless, menial task.

“I get to sit up there three times a week for an hour at a time and sit and listen to some of the greatest orators, political thinkers of various ideologies opine and argue about the issue of the day, the things that matter most to them, the things that matter most to their constituents, things that are relevant to my constituents and me,” he said.

“I actually sit there and listen,” he added. “In this line of work, I think the smartest thing you can do is listen. Listen to your constituents, listen to your colleagues, listen to experts, stakeholders. Inform yourself so you can make informed decisions.”

Fifty-eight appointed senators have served six months or less since the direct election of senators began in 1913, according to records kept by the Office of the Senate Historian. The shortest-serving senator was Rebecca L. Felton (D-Ga.), also the first woman to serve in the body. She was appointed Oct. 3, 1922, and served until Nov. 22, 1922, but was not formally sworn in until Nov. 21, giving her just one full day on the job.

Besides the death of older senators, President Obama’s 2008 election victory triggered an unprecedented wave of temporary appointments. When Obama went to the White House, he was replaced by Sen. Roland Burris (D-Ill.), who initially planned to run for a full term but backed out amid questions about his involvement in a scandal embroiling then-Gov. Rod Blagojevich (D-Ill.). Vice President Biden, who served 36 years in the Senate, was succeeded temporarily by his friend and former chief of staff, Edward E. “Ted” Kaufman. When Hillary Rodham Clinton became secretary of state, she was replaced by Sen. Kirsten Gillibrand (D-N.Y.), who won a full term in November with more than 70 percent of the vote. And when Sen. Ken Salazar (D-Colo.) stepped down to serve as interior secretary, he was replaced by now-Sen. Michael F. Bennet (D-Colo.).

When they arrive, new senators are assigned a number based on seniority that will be used forever to mark their place in history. Out of the 1,947 senators who have served since 1789, Cowan is number 1,946, just ahead of Chiesa and right behind Sen. Heidi Heitkamp (D-N.D.), according to the historian’s office.

He can still be called senator when he leaves, but Cowan said he will not use the title.

“I kind of like Mo; it’s served me well,” he said.