PORTLAND, Maine — A co-founder of Facebook who is the editor-in-chief of the liberal magazine The New Republic has offered a $100,000 matching gift to the campaign supporting same-sex marriage.

Mainers United for Marriage on Monday announced the fundraising challenge from Chris Hughes and his fiance, Sean Eldridge, president of Hudson River Ventures and senior adviser to the national organization Freedom to Marry. The campaign will have until June 7 to raise the matching funds.

Hughes owns 1 percent of Facebook, a holding valued at $850 million. The company has announced it will issue an initial public offering of stock shortly.

“Voters in Maine have a historic opportunity to win marriage at the ballot [box] in November,” Eldridge said in a press release issued by the Maine campaign. “We are encouraged by strong statewide support for the initiative and the top-notch campaign team that’s in place, and we hope that our support will motivate others to invest in the campaign. With numerous marriage equality cases heading to the Supreme Court, there is nothing more important than growing momentum and winning the freedom to marry in more states.”

Fundraising efforts will include direct mail and email solicitations, phone calls and the use of social media, David Farmer, spokesman for the campaign, said Monday.

Mainers United for Marriage will have four weeks to raise the matching funds and has set a goal of raising $25,000 per week, according to the press release. The gift is being made through Freedom to Marry’s Win More States Fund.

“Supporters of marriage in Maine have a chance to double the impact of their contributions,” Matt McTighe, the campaign manager for Mainers United for Marriage, said in the press release. “We can win in November, but we need help to continue the conversations we’re having with voters. This matching gift challenge is critical to raising the early resources we need for our campaign. Sean and Chris are strong advocates for the freedom to marry, and our campaign greatly appreciates this early support.”

McTigue said in March that he expected the campaign to raise more than the $5 million spent in the 2009 attempt to keep the same-sex marriage law from being repealed. The other side spend $2.5 million, according to a previously published report.

Mainers on Nov. 3, 2009, voted 53 percent to 47 percent to repeal the law that allowed same-sex couples to marry. It had been passed by the Democrat-controlled Legislature and signed into law by Gov. John Baldacci in the spring of 2009. After the loss at the ballot box, EqualityMaine began an outreach campaign to talk about the issue and began gathering signatures in August 2011 to put a question before voters again.

So far, Mainers United for Marriage have outraised their opponents, Protect Marriage Maine, which opposes same-sex marriage, by about 100 to 1, according to reports filed with the commission Governmental Ethics and Election Practices in Augusta. During the first quarter of 2012, supporters raised nearly $106,000 while opponents raised just under $1,700.

“We’re really at the beginning stages of fundraising, talking to people, explaining what we’re hoping to do, putting together a budget,” said Rev. Bob Emrich, spokesman for Protect Marriage Maine. “It’s been interesting this time around to get phone calls from people around the state asking what they can do, how they can help out, and offering to volunteer.”

As of March 31, Mainers United for Marriage had spent just over $18,000 while Protect Marriage Maine had not spent any of its money.

The next set of financial reports are due June 1.

Mainers United for Marriage is leading the campaign for a yes vote on a November citizens initiative that would allow same-sex couples to receive a marriage license while also protecting religious freedom. The campaign will open field offices Tuesday in Bangor, Lewiston and Portland and launch a door-to-door campaign.

Emrich said Protect Marriage Maine also plans to do some door-to-door campaigning later in the year.

The Maine Freedom to Marry Coalition, which led the petition drive that got the issue on the ballot, is made up of EqualityMaine, Gay & Lesbian Advocates & Defenders, the American Civil Liberties Union of Maine, Maine Women’s Lobby, Engage Maine and 16 other organizations.

Protect Marriage Maine is made up of the Christian Civic League of Maine and the National Organization for Marriage.