* State already outlaws same-sex marriage by statute
* Candidates for Congress, governor also being considered
By Wade Rawlins
Raleigh, N.C., May 8 (Reuters) - North Carolina voters could deal a blow to efforts across the country to expand gay marriage rights if they approve a state constitutional amendment on Tuesday to ban same-sex marriage and civil unions.
The state is the only one in the Southeast without such a constitutional prohibition, though same-sex marriage is already outlawed by statute.
The amendment is being decided amid heightened rhetoric about gay marriage from officials in the Obama administration. U.S. Vice President Joe Biden said on Sunday he was "absolutely comfortable" with allowing same-sex couples to wed, and Education Secretary Arne Duncan sai d gay marriage should be legal.
President Barack Obama has said he favors civil unions but has stopped short of supporting gay marriage.
Supporters of the proposed amendment in North Carolina, a swing state in the Nov. 6 presidential election, say it would preserve the sanctity of marriage between a man and a woman and make laws forbidding gay marriage harder to repeal.
Opponents say a ban would jeopardize health insurance benefits for unmarried gay and heterosexual couples and signal that the state is unfriendly to a diverse workforce.
Former President Bill Clinton, a Democrat, recorded a call just ahead of the vote urging North Carolinians to reject the proposed amendment.
"If it passes, it won't change North Carolina's law on marriage," he said in a message sponsored by the Coalition to Protect North Carolina Families. "What it will change is North Carolina's ability to keep good businesses, attract new jobs and attract and keep talented entrepreneurs."
A survey of 1,026 likely Democratic and Republican primary voters showed North Carolinians look poised to pass the amendment.
Raleigh-based Public Policy Polling found 55 percent of those questioned on May 5-6 supported the amendment banning gay marriage and civil unions while 39 percent opposed it. The poll had a margin of error of plus or minus 3.1 percent.
Twenty-eight states have voter-approved constitutional bans on same-sex marriages, according to the National Conference of State Legislatures.
Massachusetts, Iowa, Vermont, New Hampshire, Connecticut, New York and the District of Columbia allow gay and lesbian nuptials. Maryland, New Jersey and Washington state passed laws this year approving same-sex marriage, but Governor Chris Christie vetoed New Jersey's law and opponents of Maryland's and Washington's are threatening ballot initiatives to overturn those laws.
OTHER BALLOT ISSUES
North Carolina voters also will consider candidates for gubernatorial and congressional races.
The state's 13-member congressional delegation currently includes seven Democrats and six Republicans, but Republicans see an opportunity to pick up seats with the retirement of two Democratic incumbents.
Congressmen Brad Miller and Heath Shuler opted to retire rather than run in new districts redrawn by the Republican-controlled legislature.
The race for Miller's 13th congressional district features a Republican primary contest between George Holding, a former U.S. attorney, and former Raleigh mayor Paul Coble. While a federal prosecutor, Holding's office built the criminal campaign finance case against former U.S. Senator John Edwards, a Democrat.
Coble has accused Holding of wasting taxpayer money to advance his political career, saying the prosecution of Edwards over donor money used to hide his pregnant mistress during his 2008 presidential bid was political - a charge that Holding's campaign rejected in television ads.
Democratic Governor Bev Perdue's surprise decision in January not to seek a second term left Democrats scrambling to raise money and name recognition in the few months before Tuesday's primary.
The leading Democratic candidates are Walter Dalton, the state's lieutenant governor; Bob Etheridge, a former seven-term congressman and state superintendent of public instruction; and Bill Faison, a state representative.
A candidate will need 40 percent of the vote to claim the nomination without a runoff. The winner will face the presumptive Republican nominee Pat McCrory, a former Charlotte mayor who has minor primary opposition and has focused on building his campaign war chest.
McCrory, who lost to Perdue four years ago, has raised more than $3 million for his 2012 campaign, according to reports filed with the North Carolina Board of Elections. Dalton has raised $1.4 million, the most among Democratic contenders. (Reporting by Wade Rawlins; Writing By Colleen Jenkins; Editing by Vicki Allen)