UPDATE 3-Massachusetts gov. backs naming Kennedy successor

(Adds quotes, details, background)

* Massachusetts governor supports interim appointment

* Governor sets Jan. 19 special election

* Republicans outnumbered in both legislative chambers

By Ross Kerber

BOSTON, Aug 31 (Reuters) - Democrats pushed on Monday to fill the U.S. Senate seat left vacant by the death of Edward Kennedy, with the Massachusetts governor throwing his support behind a proposal to name an interim successor and help Democrats retain a critical margin in the chamber.

Massachusetts lawmakers said they would hold a public hearing on Sept. 9 to discuss changing state law so the Democratic governor could make a temporary appointment to the Senate, which is debating a $2.5 trillion healthcare overhaul.

Under current law, the seat would be vacant until a Jan. 19 special election that Governor Deval Patrick set on Monday to fill the remaining three years of Kennedy's term.

Kennedy's death deprives Democrats of a critical 60th vote in the 100-seat Senate which, if they are united, would allow them to overcome Republican procedural roadblocks to the healthcare overhaul urged by President Barack Obama.

Kennedy, a longtime champion of healthcare reform, made a plea before he died last week of brain cancer that state law be changed to allow a replacement to be named quickly.

The U.S. Senate Finance Committee has set a target date of Sept. 15 to reach agreement on healthcare legislation, although it is unclear if that will be met. Obama has said he wants healthcare overhaul, a top priority of his presidency, approved by the end of the year.

After Kennedy's burial on Saturday, supporters of his bid for an interim successor wasted little time setting the wheels in motion.

Patrick told a a news conference on Monday he would work with the Democratic-controlled Legislature on a bill to authorize an interim Senate appointment as Kennedy had urged.

"This is the only way to ensure Massachusetts is fully represented until voters elect our next senator in January, " Patrick said.

The current state law was enacted in 2004 in a effort by state Democrats to prevent then-Governor Mitt Romney, a Republican, from naming a successor if Democratic Senator John Kerry won his bid for the U.S. presidency. Kerry went on to lose the 2004 election to President George W. Bush.

Republican officials in Massachusetts have opposed changing the law, but they are greatly outnumbered in both chambers.