The Senate is packed with people who spend millions of dollars to win and keep their seats every six years. That makes Sen. William “Mo” Cowan (D-Mass.) a rare breed. He’s a six-month Senate seatwarmer with no desire to stick around.

Sen. Mo Cowan (D-Mass.), right, with Massachusetts Gov. Deval Patrick (D), in Boston in January. (Boston Globe via Getty Images)

Cowan is getting ready to pack up and leave to make way for whoever wins Tuesday's special election in Massachusetts to replace former Sen. John F. Kerry (D-Mass.)

In a profile in The Washington Post, Cowan describes his six months in the Senate and how he thoroughly enjoyed them. The story also notes that Cowan is one of several people appointed in recent years to fill Senate vacancies, either because of the death of longtime senators or the political promotions of others.

Most recently, Sen. Jeff Chiesa (R-N.J.) succeeded the late Sen. Frank Lautenberg (D-N.J.), but Chiesa will leave in mid-October after a special election.

Sen. Jeff Chiesa (R-N.J.). (Julio Cortez/AP)

Cowan and Chiesa join a long list of appointed senators who've served for six months or less, according to records kept by the Office of the Senate Historian.

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 ever was Sen. Rebecca L. Felton (D-Ga.), also the first woman to serve in the body. She was appointed on Oct. 3, 1922 and served until Nov. 22, 1922, but wasn’t formally sworn in until Nov. 21, giving her just one full day on the job. In terms of actual days served, Louis Wyman (R-N.H.) served just four days, from Dec. 31, 1974 to Jan. 3, 1975, in the midst of a disputed election.

View the list of appointed senators who served six months or less, below:

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