By Matthew Bigg

BOSTON (Reuters) - Senator Edward Kennedy, who has advanced brain cancer, urged Massachusetts leaders to change state law so it could quickly fill a vacancy in the Senate as it decides on an overhaul of U.S. healthcare.

Kennedy sent a July 2 letter suggesting the governor be allowed to appoint an interim U.S. senator, should one be needed, rather than wait up to five months for a special election.

A spokesman for the 77-year-old Kennedy said on Thursday the letter was not related to any deterioration in his health. The long-time Massachusetts Democrat did not attend the funeral of his sister, Eunice Kennedy Shriver, last week.

Under Massachusetts law, once a Senate seat becomes vacant there is a 145- to 160-day period before a special election is held, opening a potential five-month gap during which Massachusetts would only have one U.S. senator.

"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," Kennedy urged state leaders in the letter.

Kennedy is a lifelong advocate of reforming U.S. healthcare but illness has limited his participation in President Barack Obama's drive to revamp the $2.5 trillion sector.

Democrats need all of their party's senators to muster 60 votes in the U.S. Senate, which is crucial to blocking any Republican maneuvers to stall the legislation.

"It (Kennedy's vote) is quite critical because right now the Democrats have been absent his vote and given the situation votes in the Senate are tight," said Leighton Ku, professor of health policy at George Washington university.

"He has also been a leader on healthcare issues for almost 50 years and people recognize his strength on that," Ku said.

Kennedy sent the letter to Massachusetts Governor Deval Patrick, State Senate President Therese Murray and State House Speaker Robert DeLeo.

Kennedy also asked the governor to gain a commitment from whomever he named on an interim basis not to run in the special election to assure a level playing field for the contestants.

"He believes strongly that Massachusetts should elect their senator. But he also believes we shouldn't go without in the meantime," said a Kennedy spokesman.

(Additional reporting by Toni Clarke, editing by Vicki Allen)