The note has been sent to Gov. Deval Patrick and the state's Senate president and House speaker at a time when Congress is considering an overhaul of the nation's health care system, a life cause of Kennedy's.
The letter acknowledges the state changed its succession law in 2004 to require a special election within five months to fill any vacancy. At the time, legislative Democrats were concerned then-Republican Gov. Mitt Romney would be able to fill any vacancy created as Democratic Sen. John Kerry ran for president.
But Kennedy writes "it is vital for this commonwealth to have two voices speaking for the needs of its citizens and two votes in the Senate during the approximately five months between a vacancy and an election."
While Democrats hold a potentially filibuster-proof margin in Congress, the outcome of a health care reform bill could hinge on a single vote.
The letter was sent Tuesday, but Kennedy aides insist there is no material change in his condition since he was diagnosed with a malignant brain tumor in May 2008.
from last week's funeral for his sister, Eunice Kennedy Shriver, prompted a flurry of questions about his own health.
In his letter, Kennedy suggests the governor ensure the fairness of any appointment to replace him by seeking an "explicit personal commitment" his appointee will not seek the position on a permanent basis.
Read Kennedy's letter to Gov. Deval Patrick and state congressional leaders
Despite speculation that Kennedy's wife, Vicki, is interested in the seat, family aides have said she is not interested in replacing her husband either temporarily or permanently.
Amid similar speculation about a Senate vacancy last fall, when Kerry was under consideration for secretary of state, Senate President Therese Murray was adamant that the law not be changed. After recent inquiries from The Associated Press, aides to both Murray and House Speaker Robert DeLeo said they are unlikely to back any change.
Aides to both leaders say an election was more democratic than a gubernatorial appointment, and they cited the legal and political problems that plagued former Illinois Gov. Rod Blagojevich and New York Gov. David Paterson when they filled vacancies for President Barack Obama and former Sen. Hillary Rodham Clinton.
In a joint statement to The Boston Globe, which first reported news of Kennedy's letter, both Murray and DeLeo were noncommittal.
"We have great respect for the senator and what he continues to do for our commonwealth and our nation. It is our hope that he will continue to be a voice for the people of Massachusetts as long as he is able," they said.