The North Carolina House voted Monday to allow a May referendum on whether to enshrine a ban on gay marriage in the state constitution.

North Carolina already has a law defining marriage as being between a man and a woman, but supporters say making the ban part of the state constitution will make legal challenges more difficult. North Carolina is the only state in the Southeast without a constitutional ban on same-sex marriage.

The timing of the ballot measure is significant. Critics of the measure had charged that the Republican-led legislature would schedule the referendum for next November to try to boost conservative turnout during next year's presidential election. President Obama won North Carolina in 2008 by a thin margin.

But House Speaker Thom Tillis (R) told reporters Monday he is “reasonably confident” the referendum will go to voters in May – a signal, he said, that the legislature was not playing politics. The May date, however, coincides with the Republican primary and could boost conservative candidates like Texas Gov. Rick Perry or Rep. Michele Bachmann of Minnesota.

“Putting it in the May primary puts the [gay marriage] debate within the Republican Party, so it’s likely to solidify the party’s rightward tilt,” says Ferrel Guillory, director of the Program on Public Life at the University of North Carolina in Chapel Hill.

The bill now goes to the Senate for approval.

The amendment is worded to protect businesses in the state that may offer health and life-insurance benefits to domestic partners, but it threatens state and municipal government employees whose partners may currently be enjoying the same privileges.

State polling suggests referendum is not necessarily a slam-dunk.

Some 61 percent of North Carolinians say gay marriage should remain illegal, but 55 percent would vote against a constitutional amendment, according to a poll released last week by Public Policy Polling, a Raleigh, N.C., firm that often works for Democrats. Republican voters are split, with 47 percent saying amending the state constitution goes too far.

The challenge for Republicans is showing voters why the state needs to legislate gay marriage beyond the existing ban, says Dean Debnam, president of Public Policy Polling.

“It’s pretty simple: North Carolinians don’t support gay marriage, but they also don’t think this constitutional amendment is necessary,” Mr. Debnam said in a statement.

Adds Professor Guillory: "There’s a strong argument to be made that this amendment trifles with the constitution. It affords those who are looking for another rationale to vote against this bill and to disentangle themselves from the issue."

If the measure succeeds and the referendum is approved next year, North Carolina will become the 31st state to prohibits same-sex marriage in its constitution, according to the National Conference of State Legislatures. Civil unions or gay marriages are legal in nine states and the District of Columbia.