Passage of the health-care initiative could rest on a handful of Senate votes, and Kennedy's allies, such as Ron Pollack of Families USA, say the senator's move reflects his commitment to making the overhaul law.
"If, unfortunately, he does not live to see the key vote in the Senate," Pollack said, "he wants to make sure that his succession is handled in such a way that his legacy is truly achieved."
Kennedy's letter, dated July 2, was sent to political leaders this week. He does not address his cancer, which has required surgery, chemotherapy and radiation.
"For almost 47 years, I have had the privilege of representing the people of Massachussets in the United State Senate," Kennedy wrote. He said serving "has been — and still is — the greatest honor of my public life."
Democrats control both chambers of the Massachusetts legislature by wide margins, but would face resistance to Kennedy's proposal from the GOP ranks, said Senate Minority Leader Richard Tisei, the top Republican in the state Senate.
The Democratic-controlled legislature changed the state's succesion law in 2004, denying then-governor Mitt Romney, a Republican, the ability to fill a Senate vacancy as Democratic Sen. John Kerry competed for the White House.
"I feel terrible for Sen. Kennedy," Tisei said. But he said it would be "very hard for the Democratic majority to change the law when they so passionately advocated" to alter it in 2004. "We shouldn't be changing the laws based on personal circumstances."
Murray and DeLeo, the legislature's top Democrats, issued a joint statement saying they hoped Kennedy would "continue to be a voice for the people of Massachusetts as long as he is able."