By Edith Honan

TRENTON, New Jersey (Reuters) - The New Jersey state Senate prepared to vote on a bill to legalize gay marriage on Monday, despite a promised veto from Gov. Chris Christie, one of the country's leading Republican figures and a possible vice-presidential candidate.

The state's Democratic leadership, which controls both houses of the state legislature, has made gay marriage a top priority this session, saying New Jersey's civil union law does not adequately protect same-sex couples.

Debate in the Senate, which rejected a similar measure two years ago, was under way.

The bill was expected to pass on the Senate on Monday and the state Assembly on Thursday, but the Democratic-controlled state legislature was unlikely to have the two-thirds majority needed to override Christie's promised veto.

The New Jersey Senate vote comes on the same day Washington state Gov. Christine Gregoire planned to sign a bill making Washington the seventh U.S. state to legalize same-sex marriage.

Gay marriage is already allowed in New York, Connecticut, Iowa, Massachusetts, New Hampshire and Vermont plus the District of Columbia.

A Gallup poll last year found for the first time a majority of Americans believe gay marriage should be legalized, with 53 percent in favor.

New Jersey's high court ruled in 2006 same-sex couples must be guaranteed the same rights as heterosexual married couples but left it up to the legislature to write the law. The legislature approved a civil unions law for same-sex couples later that year.

Gay couples argued the state had created a two-tier system that was inherently unequal.

A new lawsuit, which challenges New Jersey's civil union law, is pending.

(Editing by Daniel Trotta)