Rod Zimmer's departure, made known in a letter to the governor general, was circulated around the Senate on Tuesday, and comes against the backdrop of an ongoing scandal over improper expense claims.
A spokesman for Liberal Sen. Jim Cowan, the leader of the Opposition in the Senate, says Zimmer's resignation is strictly health related, and has nothing to do with the ongoing expense review.
His resignation letter was submitted late Friday and took effect immediately, according to a copy of the notice, obtained by The Canadian Press.
Zimmer, 70, has represented Manitoba in the upper chamber since being appointed in 2005 by former prime minister Paul Martin.
A long-time Liberal fundraiser and executive at CanWest Corp. and Manitoba Lotteries, Zimmer has been in and out of hospital several times this year, most recently in May with a respiratory ailment.
He also spent nearly three weeks in hospital last spring battling pneumonia.
Sources in the Senate said the timing of the resignation took them by surprise, but it was widely expected that Zimmer would eventually give up his seat because of his continuing poor health.
Prior to being appointed to the Senate, he was diagnosed in 2003 with throat cancer, but bounced back.
A spokeswoman for Liberal Leader Justin Trudeau says Zimmer notified him ahead of time about his intention to leave.
"He informed Mr. Trudeau and Sen. Cowan last month that he would be stepping down as a senator due to health reasons," Mylene Dupere said in an email.
"The Liberal caucus wishes him well."
Zimmer raised eyebrows when he married Maygan Sensenberger, 23, an aspiring actress who pleaded guilty last year to causing a disturbance while the pair were on board an Air Canada flight bound for Saskatoon.
In subsequent interviews last fall, Sensenberger described the incident as overblown, but did not elaborate.
Sensenberger was arrested after some passengers on the same flight reported that she was arguing with Zimmer and threatening to slit his throat. Others, however, said she became upset after Zimmer began experiencing tightness in his chest and she thought he was having a heart attack.
Zimmer supported his wife throughout the court proceedings, and insisted he never felt threatened by her.
His resignation leaves four vacancies in the Senate, and diminishes the ranks of the Liberal Opposition to 34 seats. The governing Conservatives hold 60 seats and there are seven independents, including the four at the centre of the expense scandal.
The Harper government says it is intent on reforming the Senate.
Last week, in a brief filed with the Supreme Court, government lawyers argued the federal government does not need the approval of the provinces to proceed with the overhaul, and can abolish the upper chamber even without the unanimous consent of all premiers.
The government referred the question of Senate reform to the high court last February in a bid to proceed without having to reopen the Constitution.
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