MADISON, Wis. -- A new statewide poll gives supporters of a state constitutional amendment defining marriage a narrow advantage.

According to the WISC-TV-sponsored survey, 48 percent of those polled said that they would vote for the amendment if the November election were held today. About 40 percent oppose the measure and 12 percent said that they are undecided.

The telephone poll randomly called 600 likely voters over a three-day period from Aug. 14 to 16. The margin of error in the survey is 4 percent. Besides the marriage amendment, the poll also asked questions about the races for governor and attorney general.

University of Wisconsin political science professor Charles Franklin said that the survey results suggest the outcome in November will be close and that is appears to enjoy less support in the state than the idea does in other parts of the country.

"It shows it's going to be a closer vote here in Wisconsin than it has typically been (in other states)," Franklin said. He said that in most other states, polls show support for similar questions is about 60 percent.

Franklin said that he also believes the narrow gap determined by the survey results suggests the strength of Fair Wisconsin, the group heading the effort to defeat the amendment.

In the poll, likely voters were asked: "Would you favor or oppose a state law which would constitutionally limit marriage between a man and a woman and would ban civil unions and other arrangements for unmarried couples that are 'substantially similar' to marriage?"

The survey found that Wisconsin men were more likely to favor the ballot question than women and there were distinct differences on party affiliation. Among Democrats, the amendment isn't supported -- 39 percent would vote for it, but 52 said that they wouldn't. Nine percent were undecided.

It's the same story with independents, as 38 percent said that they support the change in the constitution, 50 percent said that they would oppose it and 12 percent said that they haven't made up their mind.

The results were far different with Republicans, as the idea had overwhelming support. About 65 percent of Republicans said that they would vote for the amendment and only 20 percent said that they wouldn't. Fifteen percent said that they haven't decided yet.

The poll also found that support for the amendment has support in most of the state's major metropolitan areas with the exception of Madison. Here, opponents of the amendment outnumber supporters by a margin of 45 percent to 39 percent.

Franklin said that he believes the second question on the November ballot will be the key to the amendment's success come November. He said how voters interpret "substantially similar" to marriage could greatly influence the outcome.

"If voters see this as a vote on gay marriage, it'll probably pass by a substantial margin," said Franklin. "If they see it as an intrusion into contracts and other kinds of benefits among unmarried but heterosexual people, then supports going to go down quite a bit."

This poll is the first in a series of four polls leading up to the November election.