The latest polling in North Carolina suggests the state’s voters will choose to ban same-sex unions via referendum today, even as popular opinion nationwide trends in support of gay marriage.

North Carolina already bans gay marriage by statute. Amendment One goes beyond confining marriage to heterosexual couples; it amends the constitution to also makes marriage between a man and a woman “the only domestic legal union” recognized in the state.

Opponents argue that the ban will endanger legal benefits for unmarried couples, gay and straight, as well as protections for victims of domestic violence. Retiring Gov. Bev Perdue (D) called the vote on Amendment One “our Rosa Parks moment” in an interview with NBC’s Chuck Todd on Tuesday.

The issue is clearly driving considerable enthusiasm — and turnout. Nearly half a million people voted early in North Carolina — more than did in the 2008 presidential election.

A poll this week from the Democratic firm Public Policy Polling found the amendment winning 55 to 39 percent — only a slight change from a week earlier when the amendment led 55 to 41 percent.

Fifty-three of voters in the state support either gay marriage or civil unions, but only 46 percent of voters realize the amendment would ban both, according to PPP.

“Opponents of the amendment had an uphill battle in convincing voters that it was anything other than a referendum on gay marriage, even though it does go a lot further than that,” said PPP pollster Tom Jensen.

Those poll numbers are somewhat remarkable given that the amendment’s opponents have enjoyed nearly a million dollar fundraising advantage.

“Most of our money came in the final six weeks,” said Jeremy Kennedy, campaign manager for the anti-amendment Coalition to Protect North Carolina's Families. “We needed more resources early on.”

Amendment opponents argue that the high early turnout suggests a possible upset, as polls forecast a smaller electorate. Supporters argue the opposite.

“Polling shows that despite racial differences, despite party differences, across gender lines, North Carolinians are supportive of the definition of marriage as between one man and one woman,” said Rachel Lee, spokeswoman for Vote FOR Marriage NC.

President Obama, who supports civil unions but not gay marriage, has stayed out of this fight — and he canceled a scheduled trip to the state today. Although his Administration opposes the amendment, the president himself has been quiet on the subject, frustrating some of his North Carolina supporters.

If the amendment passes, Human Rights Campaign and other groups “will look into all options to overturn it,” said HRC deputy press secretary Paul Guequierre. 

Voters in 28 states have instituted constitutional bans on same-sex marriages, according to the National Conference of State Legislatures. Eleven of those bans were instituted in 2004; some Democrats say anti-gay initiatives helped drive turnout for then-President George W. Bush.

But support for these measures is declining over time. In 2004, gay marriage bans passed on average with 71 percent of the vote, but in 2008 the average was 57 percent, according to HRC. However, in Southern states like North Carolina bans tend to pass by wider margins.

Polls close at 7:30 p.m. eastern time. A competitive Democratic gubernatorial primary to replace the retiring Perdue is also bringing voters to the polls. 

Read more from PostPolitics

RNC Hispanic roundtable backfires on Romney

Dick Lugar will lose. Did he have to?

She the People: Hillary Clinton, barefaced and refreshing