Matt McTighe, campaign manager for Mainers United for Marriage, a coalition of groups advocating gay marriage, said his side had more time to make its case this year and was playing offense, not defense.

"Normally, when marriage comes up on the ballot, it's in response to a court case or legislative action and at most -- as in 2009 here and in Maryland and Washington -- you have six months to really go out and defend whatever that court or legislature did," Mr. McTighe said. "Maine is the first state to proactively bring this initiative to voters, and that's allowed us to control our own fate."

The campaign's centerpiece is a canvassing effort aimed at undecided voters. Mr. McTighe said a personal approach was the most effective way to convey the message. The coalition has conducted 100,000 "conversations" in two years, he said, with perhaps one in every five people open to changing his or her views.