A majority of Virginians support a proposed constitutional amendment that would ban same-sex marriage and civil unions, although voters split on the measure when presented with interpretations of its potential impact, according to a new Washington Post poll.

Fifty-three percent of likely voters said they would vote for the amendment, and 43 percent would oppose it, the poll found, indicating that three weeks before Election Day opponents still have a long way to go to make Virginia the first state in the country to defeat a same-sex marriage amendment.

The only part of the state to oppose the measure was Northern Virginia, where voters rejected it 55 percent to 42 percent, further evidence that the Washington suburbs have become a political and social world apart from the rest of Virginia. Respondents in the rest of the state backed the measure 58 percent to 38 percent, according to the survey, conducted over three days last week.

Despite the overall results, the poll provided some hope for opponents of the measure. Their chief argument is that the language of the amendment is too broad and would endanger contracts between unwed heterosexual couples. Supporters contend that the measure is limited to declaring that same-sex marriages would never be approved or recognized in Virginia.

When respondents were read the arguments on both sides of the question, enough voters showed a willingness to reconsider that the gap narrowed to a virtual tie -- 48 percent said they supported the measure and 47 percent opposed it, within the poll's margin of error of three percentage points.

"I really do think that we need an amendment like this that defines marriage," said Ross Williams, 50, a salesman from Annandale, who considers himself a Republican and a likely voter for Sen. George Allen (R) in his reelection bid Nov. 7. "But we don't need to pass something that we don't know what it's going to do. I wouldn't call myself an undecided voter yet . . . but I am going to read the wording and make up my mind about what it's going to do before I vote."

The poll also found that Virginians are virtually split over whether gay couples should be able to form civil unions, which would give them health insurance, inheritance benefits and other legal rights of married couples. The poll found that 48 percent believe gay couples should be allowed to engage in civil unions and 47 percent do not. The amendment would constitutionally ban civil unions between gay couples, although they are already illegal in Virginia.

"We've been seeing this for a long time now: When people read the entire question and think about what it means, they vote no," said Claire Guthrie Gastanaga, campaign director for the Commonwealth Coalition, the group organizing opposition to the amendment.

Victoria Cobb, executive director of the Family Foundation, said that she believed the amendment would pass and that voters understood that the ballot question was simply about defining marriage. She said an internal poll conducted by the group found strong support for the measure.

"We are very confident that Virginia is going to join the other 20 states that have passed constitutional amendments," she said. "Virginians know this is about marriage between one man and one woman, and we have complete confidence that when they step into the voting booth, they will vote for traditional marriage."

Several political scientists who have studied state ballot measures said the polling data from Virginia appeared to defy expectations, given the commonwealth's reputation as a conservative state.

"This is quite a surprise," said Daniel A. Smith, an associate professor of political science at the University of Florida. "In an ostensibly conservative state like Virginia, you'd expect to see the numbers up around 60 or 70 percent."

The lower numbers in Virginia reflect a national trend of weakening support for state efforts to ban same-sex marriage, several experts said. Twenty states have passed similar measures since 1998, many with about 75 percent support. The lowest level of support an amendment received was 57 percent in Oregon in 2004.

But this year, poll results in several states with similar ballot measures show weaker support than in 2004, when 11 states passed constitutional amendments. Polls in Colorado and Wisconsin show results similar to Virginia's; poll results in South Dakota are mixed.

John C. Green, a senior fellow at the Pew Forum for Religion and Public Life, said the momentum for such amendments at the ballot box has been hurt by recent court cases that have upheld bans on same-sex marriages.

"Two things seem to be going on," Green said. "Opponents [of same sex marriage amendments] are much more organized than they were two years ago. And if you look at what judges have been doing over the past year . . . they've been supporting the definitions of traditional marriage. So some of the edge is off."

The constitutional amendment received its strongest support from Republicans, blacks and those who say they go to church more than once a month, reflecting national trends.

"My religion teaches that marriage is between a man and a woman, and quite frankly, that's all I need to help me understand how I need to vote," said Sandy Ledford, 57, a housewife from Botetourt County, in the state's rural southwest. "It's pretty simple for me."

"I'm not a homophobe," said Charles Wortham, 60, a dentist from Hanover County, outside Richmond. "My view is simply that marriage is there to establish a legal process for the procreation of children. It's Mother Nature. Same-sex couples can't naturally reproduce, so it doesn't seem like they should be able to marry like a traditional family."

Wortham said that he had heard the arguments raised by opponents regarding the measure's unintended consequences but that he didn't believe the claims. "I don't think any of that stuff is going to happen," he said.

The Washington Post poll is based on telephone interviews with 1,004 randomly selected Virginia likely voters and was conducted Oct. 10-12.

The poll results generally hewed to party lines: Seventy-five percent of Republicans back the amendment, and 59 percent of Democrats oppose it. Independents were nearly split, with 49 percent against it and 45 percent for it.

Seventy-three percent of people who also said they would vote to reelect Allen backed the amendment, and 23 percent said they'll vote against it. Two-thirds of those who support Allen but oppose the amendment are women.

Among those who said they would vote for Democrat James Webb for U.S. Senate, 67 percent opposed the measure and 31 percent supported it. Allen favors the amendment, and Webb is opposed because he thinks the language goes beyond prohibiting same-sex marriage. Webb believes marriage is between one man and one woman.

The only group to significantly cross party lines was blacks. In the poll, blacks supported Webb by 81 percent to 11 percent, but they favored the amendment 61 percent to 34 percent. "I don't believe in gay relationships; I just don't believe that they are right," said Aaron Moore, 26, from Chesapeake, who added that he follows the Pentecostal faith and will vote for Webb. "Even though I'm a Democrat, it's just something that I disagree with most Democrats on."

Whites supported the measure 51 percent to 45 percent, the poll found.

Opponents of the measure include Democrats, those who said they do not attend church often and political independents. "These kinds of rights, being able to get married, they should be open to everybody," said Beverly Gordon, 51, a teacher from Hampton. "Voting, access to schools, marriage -- these are all things that people should have equal ability to have."

Others said they were concerned about unintended consequences. The amendment "doesn't specify anywhere that it's talking only about homosexual couples," said Faye Talhame, 49, a real estate agent from Loudoun County. "This goes way too far. Who knows what it would do? Why do we want to pass such a thing?"

Polling director Jon Cohen, database editor Dan Keating and staff writer Robert Barnes contributed to this report.