(Reuters) - The New Jersey Assembly passed legislation legalizing same-sex marriage on Thursday, sending the bill to Republican Gov. Chris Christie, a possible vice-presidential candidate who has promised to veto the measure.

The Assembly vote of 42 to 33 followed Senate approval on Monday and increased the scrutiny on Christie, a supporter of Republican presidential hopeful Mitt Romney who has attempted to deflect controversy by saying the matter should be settled by voters in a referendum.

Both chambers lack the two-thirds majority needed to override a veto by Christie blocking gay marriage, a hot-button social issue that is gaining traction in the 2012 U.S. presidential election campaign.

In Maryland, the House of Delegates delayed until Thursday evening its debate on the contentious Civil Marriage Protection Act sponsored by Gov. Martin O'Malley, who is seen as a rising star in the Democratic Party.

The bill was passed by two House committees on Tuesday, but it remains unclear if O'Malley and his allies have pulled together the 71 votes needed to clear the full House. Comparable legislation failed to pass a year ago when a number of African-American Democrats joined Republicans in opposing the bill.

O'Malley, a gay marriage supporter, has attempted to sway the black lawmakers, many of whom are hesitant to back an issue opposed by much of Maryland's black clergy.

The action in the Maryland and New Jersey statehouses follows passage of a same-sex marriage bill in Washington state, where Governor Christine Gregoire signed it into law on Monday, but it will not take effect until at least June.

Opponents are working to gather signatures for a ballot initiative in November that would block the law. One group of repeal advocates is also seeking a referendum in early March to put the issue directly to the state's voters.

A federal appeals court in California earlier this month overturned that state's gay marriage ban, enacted through a 2008 ballot initiative. That sets up a possible showdown in the U.S. Supreme Court over the matter.

So far, six states are able to perform same-sex weddings: New York, Massachusetts, Connecticut, New Hampshire, Vermont, and Iowa. Washington is poised to become the seventh, although the law could be blocked by a ballot initiative.

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

The initiative now shifts to Christie, who has 45 days to veto the bill but has vowed to do so immediately.

Democrats do not currently appear to have enough votes to override the veto with a two-thirds majority, though they have until the end of 2013 to try.

(Additional reporting by Alice Popovici in Annapolis, Md; Editing by Daniel Trotta and Anthony Boadle)

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