AUGUSTA — National opponents of same-sex marriage say they will be actively involved in Maine and the three other states that probably will vote on the issue this fall.

During a conference call Tuesday, Brian Brown, president of the National Organization for Marriage, said his organization will “be very focused on Maine,” but he would not speculate how much the group would spend. They spent nearly $2 million during the 2009 campaign to overturn a gay marriage law in Maine, and has teamed up with the Christian Civic League to fight the ballot question this time around.

“We view (Maine) as a key state,” he said. “We’ll see how much money we have to spend. We haven’t determined how much we can devote to each state.”

They also plan to target President Barack Obama in key swing states, such as Ohio and Florida, saying his recent expression of support for gay marriage will make him a one-term president.

“What we will see is the presidential election being a proxy for the survival of traditional marriage in America,” said Frank Schubert, political director for the National Organization for Marriage.

Schubert, who was instrumental in directing the 2009 campaign in Maine, joined with Brown for a conference call Tuesday with reporters from around the country. The organization has endorsed presumptive Republican presidential nominee Mitt Romney, and Schubert says he believes Romney’s support for traditional marriage mirrors the opinion of most Americans.

Maine is one of four states where ballot measures are almost certain to greet voters in the fall. Following a 2009 defeat at the polls, gay marriage supporters in Maine launched a citizen initiative last year to ask voters to allow gay and lesbian couples to marry.

It will be the first time anywhere in the country that supporters of gay marriage have gone directly to voters without getting legislative or judicial approval first.

David Farmer, spokesman for Mainers United for Marriage, said polling data and anecdotal evidence gathered through conversations with voters have supporters convinced that Maine voters are ready to approve same-sex marriage.

“The tide is turning on NOM and Schubert and their scare tactics,” he said. “We believe Maine has the very best chance of winning.”

Three recent polls on gay marriage in Maine have shown support ranging from 54 percent to 58 percent.

Schubert said Tuesday that polls on gay marriage are notoriously inaccurate because the questions are often not asked in an objective way and because people are afraid to express how they really feel about the issue. He said support for traditional marriage is consistently understated by 7 percentage points in most polls.

“When people go to the polls, they vote to protect marriage,” Brown said.

Other states where gay marriage is likely to be on the ballot:

* In Maryland, Gov. Martin O’Malley, who spoke in Maine last weekend at the Democratic convention, signed a bill into law in March allowing gay marriages. However, gay-marriage opponents have turned in twice the number of signatures needed to call for a citizen vote in November, Schubert said.

* In Washington state, groups are scheduled to turn in signatures today to call for a public vote following enactment of a gay marriage law in February. They, too, say they have well more than the number needed to qualify for the ballot, according to the Associated Press.

* In Minnesota, lawmakers will ask voters if they want to approve a constitutional amendment that defines marriage as the union of one man and one woman. If that happens, Minnesota will be the 32nd state in the country to enact a constitutional prohibition of gay marriage.

Last month, voters in North Carolina approved a constitutional ban with 61 percent support.

So far, gay-marriage supporters in Maine have raised far more money than opponents, according to campaign finance reports filed last week. The major political action committee in support of gay marriage, Mainers United for Marriage, has raised $464,722 in cash contributions and received $81,461 in in-kind contributions, for a total of $546,183.

The leading in-state opposition group, Protect Marriage Maine, has received $11,439 in cash and $3,506 among in-kind donations, for a total of $14,945. Opponents have said they are counting on a Father’s Day fundraiser at churches across Maine to give them a financial boost.

Farmer said 2,000 Mainers have donated money to support same-sex marriage, showing a high level of enthusiasm that will help carry them through the election. Obama’s support has raised awareness of the issue and made it easier to have the type of one-on-one conversations that are changing voters’ minds, he said.

“The biggest thing we’ve seen is people are engaged with the issue,” he said. “We haven’t seen any negative reactions or backlash.”

Schubert said studies have shown that moral issues such as same-sex marriage draw out voters who don’t normally participate in gubernatorial or presidential elections. The president’s support has served to motivate opponents, he said.

“Each of these states will be very different,” Schubert said. “I believe traditional marriage will prevail in all four.”