Sen. Edward M. Kennedy , who is battling brain cancer, has asked Democratic Massachusetts leaders to change state law so that his successor could be appointed rather than wait five months for a special election.

The timeliness of his successor could be important to votes on health care, climate change and other significant legislation pending this fall.

As reported Thursday by the Boston Globe, Kennedy this week sent Gov. Deval Patrick , state Senate President Therese Murray and state House Speaker Robert DeLeo a letter dated July 2 asking them to change the law for a temporary gubernatorial appointment until a special election is held.

Kennedy specifically said the governor should obtain a commitment from the appointee to serve temporarily and not participate in the special election, to ensure a fair contest.

“The senator had planned to deliver the letter at some point in the future, but decided to deliver it this week to avoid the potential of his and others’ conversations about a change in the law leaking to the press without his views being publicly known,” said spokesman Anthony Coley.

“He’s hopeful that when the legislature returns in September, the governor and leaders in the statehouse can work together to change the law through the regular course of legislative business to close that ‘gap in representation,’ ” Coley said.

Senate Democrats and the two independents who align with them control 60 votes — enough to overcome a filibuster — but the continued absence of Kennedy and ailing 91-year-old Sen. Robert C. Byrd , D-W.Va., means that they are unable to reach that number.

When asked about the importance of Kennedy’s continued absence, a Democratic aide replied, “With an agenda full of important issues like health insurance reform, energy and climate change legislation and countless others, it’s always important have enough votes to overcome Republican filibusters.”

So far this year, Kennedy has voted on nine of 270 Senate roll call votes, most recently on April 27 to invoke cloture on a bill (S 386) to toughen anti-fraud laws. He also voted March 26 on an expansion (HR 1388) of national service programs that was named in his honor; on April 2, to adopt the Senate’s version of the fiscal 2010 budget resolution (S Con Res 13) and to dispose of two estate tax amendments to it; and on Feb. 10 on two amendments and passage of the Senate’s version of the economic stimulus legislation (PL 111-5).

Changing State Law

In most states, the governor is authorized to appoint an interim senator when a vacancy occurs, with an election for the seat held on the next regularly scheduled congressional election day. That contest would either be the regularly scheduled election for a full term, if the departed senator’s term is expiring, or a special election to fill out the remaining years of the unexpired term.

The Massachusetts law was changed in 2004 because of the prospect that Republican Gov. Mitt Romney might appoint the successor to Democratic Sen. John Kerry , who was then running for president

Now the law calls for a special election to be held within 145 to 160 days after the seat becomes vacant.