UPDATE 2-Ailing Kennedy seeks quicker US Senate succession

* Kennedy wants senate seat filled quickly

* Massachusetts senators could be key in healthcare vote

* Local Democrats could face dilemma over Kennedy letter (Updates with reaction, quotes)

By Matthew Bigg

BOSTON, Aug 20 (Reuters) - U.S. Senator Edward Kennedy, who has advanced brain cancer, urged Massachusetts leaders to change the law so the state could quickly fill a vacancy in the Senate as it decides on an overhaul of U.S. healthcare.

Kennedy sent a 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.

The issue could be vital to President Barack Obama's drive to overhaul the $2.5 trillion healthcare sector if it enables Senate Democrats to reach a 60-seat majority and block any Republican maneuvers to stall its legislation.

But it was unclear whether state Democratic legislators would support the change and Massachusetts Republicans, while expressing respect for Kennedy, decried his proposal as hypocritical.

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 said in the letter sent to state leaders including Governor Deval Patrick.

A spokesman said the letter, dated July 2, was not related to any deterioration in the health of the 77-year-old senator who is a lifelong advocate of reforming U.S. healthcare and providing universal coverage.

Kennedy did not attend the funeral of his sister, Eunice Kennedy Shriver, last week.

"HYPOCRITICAL"

Healthcare reform is a key domestic priority for many Democrats and the state party has the power to change the law because it holds a big majority in the legislature and Governor Patrick is also a Democrat.

But analysts said the letter poses a dilemma for the state party and the Boston Globe newspaper reported that no party legislators voiced immediate approval for the change.

"There will be substantial reluctance (among Democrats) to change the law," said Joseph McEttrick, a professor at Suffolk University Law School in Boston, adding: "All politics is local."