Support for Referendum 74, the Washington state referendum on a gay marriage law, holds a 15-point lead, according to a survey released Friday.

The poll was commissioned by KING 5 TV in Seattle and conducted by SurveyUSA, which interviewed 540 Washington adults in person and by telephone.

Fifty-five percent of respondents said they support keeping the law approved by lawmakers, while 40 percent disagreed. Six percent said they were not sure.

Compared to a SurveyUSA poll released last month, the race has tightened 3 percentage points, cutting supporters' lead from 18 points down to 15.

Strong support for the measure is found in western Washington (60% in favor, 34% opposed), while strong opposition is found in the eastern half (33% in favor, 62% opposed).

With only 6 percent of respondents undecided, passage looks promising. No U.S. state has ever voted to enact marriage equality – though Washington did uphold a domestic partnership law which gave gay couples all the rights of marriage. The issue also appears on the ballot in Maryland and Maine, where polls also appear promising for supporters. And a proposed amendment to the Minnesota constitution which would define marriage as a heterosexual union is trailing in the polls by 1 percent.