AUGUSTA — While the outcome of November’s statewide referendum on gay marriage is very much up in the air, one thing is certain: Millions of dollars will be spent to influence voters.

Supporters of gay marriage got a financial boost Monday when Chris Hughes, a co-founder of Facebook, and his partner pledged $100,000 with the stipulation that other supporters match that amount.

It’s part of what’s projected to be a $5 million effort to make same-sex marriage legal in Maine. A similar amount is expected to be spent by opponents of gay marriage.

So far, eight political action committees have signed up to participate in the campaign, five of them supporting gay marriage and three opposing it.

As of April 20, groups supporting same-sex marriage had raised $230,502 and opponents had raised $2,313, according to a database compiled by MaineToday Media.

At this early stage in the campaign, political action committees are giving money to each other as both sides establish which groups will handle most of the money. The major PACs to watch are Protect Marriage Maine, which is opposed to the proposal on the Nov. 6 ballot, and Mainers United for Marriage, which supports it.

The next campaign finance reporting period ends May 29.

Among top individual contributors through April 20: Attorneys Pat Peard and Mary Bonauto gave $10,000 each, as did physician Judith Chamberlain and property manager Jim Bishop. All four gave to Mainers United for Marriage.

“The biggest thing is, (money) allows us to get our stories out to as wide a universe as possible,” said Matt McTighe, campaign manager for Mainers United for Marriage.

Among opponents of same-sex marriage, homemaker Calli Steuer gave $250 and the Christian Civic League of Maine contributed $216.

“Raising money in Maine is always difficult,” said Bob Emrich, chairman of Protect Marriage Maine. “We’re going to be appealing to the people around the country whenever we can.”

Three years ago, groups for and against same-sex marriage in Maine spent a total of $9.6 million during a campaign in which opponents of gay marriage repealed a law passed by the Legislature to allow gay and lesbian couples to marry. The vote was 53 percent to 47 percent.

Two campaigns and more than 20 other groups spent money to influence the outcome, including the National Organization for Marriage and the Roman Catholic Diocese of Portland in opposition to gay marriage, and the wealthy hedge fund manager S. Donald Sussman and the Human Rights Campaign in support of it.

Sussman is a frequent Democratic donor who is majority share owner of MaineToday Media, which owns the Morning Sentinel, the Kennebec Journal and The Portland Press Herald/Maine Sunday Telegram.

In 2009, Sussman donated $526,000 to support gay marriage in Maine, according to campaign finance reports. On the other side, the leading donor was the National Organization for Marriage, which contributed $1.9 million.

In total, gay marriage supporters spent $5.8 million, while opponents spent $3.8 million.

Gay marriage is legal now in six states and the District of Columbia, and votes on the issue are expected this fall in Maryland, Minnesota and Washington, in addition to Maine.

Emrich said last week’s vote in North Carolina approving a constitutional amendment banning gay marriage will help his side raise money.

“It shows the momentum has not changed,” he said. “People will see it as a worthwhile investment.”

Political scientists say they expect that both campaigns will raise and spend at least as much — and probably more — than they did three years ago.

Marvin Druker, a professor at the University of Southern Maine’s Lewiston-Auburn campus, said national attention will once again turn to Maine this fall, particularly because no state has ever granted same-sex couples the right to marry at the ballot box.

“There’s always that rule that if your opponent is spending money, then you should in return,” Druker said. “That’s part of what fuels it.”

Another factor is that it’s cheap to buy air time in Maine, particularly compared with other states, said Jim Melcher, a political science professor at the University of Maine at Farmington.

“Both sides are feeling very motivated,” Melcher said.

Other considerations are the improving economy, the presidential election and a sense that, nationally, more people say they support gay marriage, he said.

Along with the millions that likely will be spent on television ads, direct mailers and phone calls, there’s another element at play: turnout.

“What electorate shows up?” Melcher said.

State House Writer Susan Cover can be contacted at 620-7015 or at: scover@mainetoday.com