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 - with a wide majority in both chambers - were concerned because then-Republican Gov. Mitt Romney had the power to directly 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 and some moderate Democrats have been wavering.
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. Kennedy was initially treated with surgery, followed by chemotherapy and radiation treatment.
"For almost 47 years, I have had the privilege of representing the people of Massachusetts in the United States Senate, Kennedy wrote in his letter. He added that serving in the Senate "has been - and still is - the greatest honor of my public life."
The 77-year-old has been convalescing at his homes in Washington and in Hyannis Port, as well as a rental property in Florida, but his absence from last week's funeral on Cape Cod for his sister, Eunice Kennedy Shriver, prompted a flurry of questions about his own health.
An aide said the letter was one of several written by Kennedy in early July. Another was to Pope Benedict XVI and was hand-delivered by President Barack Obama during a visit to the Vatican.
In his succession 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.
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. One of Kennedy's nephews, former Rep. Joseph P. Kennedy II, has also been described as interested, along with a number of the state's remaining congressional members and local lawmakers.
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, respectively.
Murray is also a strong backer of Martha Coakley, the first female attorney general in Massachusetts and someone who has quietly laid the groundwork for a special-election campaign.
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.
Patrick said in a statement: "It's typical of Ted Kennedy to be thinking ahead and about the people of Massachusetts, when the rest of us are thinking about him."
Patrick was the top civil rights official in the Clinton administration, and he has argued about the importance of the public vote. But last fall he noted more than 40 other states fill congressional vacancies by gubernatorial appointment. He also cited the state's deteriorating fiscal condition as one argument to skip a special election and empower the governor to fill vacancies.
"These are always sensitive calls, but there are sensitive calls and decisions that governors have to make," he said in December.
Under the current law, the governor must call an election within 145 to 160 days of receiving a resignation letter. A primary would be held five or six weeks beforehand, reducing the time candidates would have to raise money for a campaign.
Besides Joseph Kennedy and Coakley, Democrats who might try to succeed Kennedy include Reps. Stephen Lynch, Michael Capuano, Edward Markey, James McGovern and William Delahunt.
Former Rep. Martin Meehan, now chancellor of the University of Massachusetts at Lowell, has $4.8 million in his federal campaign account, the largest sum of any potential candidate. That would give him the advantage in any special election sprint.
On the Republican side, potential candidates include Cape Cod businessman Jeff Beatty, former Lt. Gov. Kerry Healey, former U.S. Attorney Michael Sullivan and Chris Egan, former U.S. ambassador to the Organization for Cooperation and Development.