Foes and supporters of same-sex marriage are gearing up for five costly and bruising statewide showdowns in the coming months on an issue that evenly divides Americans.

It’s an election year subplot sure to stir up heated emotions — even beyond the confines of Maine, North Carolina, Minnesota, Maryland and Washington state. National advocacy groups will be deeply engaged, and advertising is likely to surface from each side that outrages the other.

“It’s crunch time,” said Brian Brown of the National Organization for Marriage, the paramount fundraiser for opponents of gay marriage. “We view it as a massive opportunity for a national referendum.”

Brown predicts same-sex marriage will be rebuffed in all five states, while gay-marriage supporters hope they can score at least a few victories and break a long losing streak. Since 1998, 31 states have had ballot measures related to same-sex marriage, and in every state the opponents ended up prevailing.

However, the most recent vote was in 2009, when Maine voters overturned a Legislature-approved law legalizing gay marriage. Maine same-sex marriages advocates turned in more than 100,000 petition signatures in January, more than enough to get it back on the ballot in November should Republican Gov. Paul LePage and the GOP Legislature choose that option rather than adopting it outright.

Gay-rights activists believe public opinion is moving inexorably in their direction, citing both national polls and policy developments such as repeal of the ban on gays serving openly in the military.

“The events of the past few years are bringing new energy and vigor to our side that allows our messaging to constantly evolve,” said Fred Sainz of the Human Rights Campaign, a national gay-rights group.

A look at the other states likely to vote on marriage this year:

In North Carolina on its May 8 primary day and in Minnesota on Election Day in November, voters will weigh in on constitutional amendments placed on the ballot by Republican-controlled legislatures that would ban gay marriage. Neither state allows same-sex marriage now, but supporters say the amendments would prevent courts from empowering same-sex couples to wed in the future.

In Maryland and Washington, foes of same-sex marriage are expected to gather enough signatures in the coming weeks to place measures on the Nov. 6 ballot that would overturn recently passed same-sex marriage laws. The laws are strongly backed by Democratic Govs. Martin O’Malley of Maryland and Christine Gregoire of Washington — both Roman Catholics — and strongly opposed by the Catholic hierarchy.

Washington may provide gay-marriage supporters with their best chance of victory in November. It has the only electorate in the nation that has voted to grant gay couples the same rights as heterosexual married couples, upholding a comprehensive domestic-partnership law in 2009.

“We can’t take anything for granted — we have to make the case,” said Evan Wolfson, president of the national advocacy group Freedom to Marry. “We believe Washington is a state we can win.”

Freedom to Marry is among numerous national organizations girding for what Wolfson calls “multimillion-dollar slugfests” in the five states. It’s launching a “Win More States” fund with a goal of raising $3 million for the campaigns.

Big contributions are likely to come from the Human Rights Campaign in support of gay marriage and from the National Organization for Marriage opposing it.

“It’s going to be a big challenge, but I think we’re up to it,” said NOM’s Brown.

Sainz said the Human Rights Campaign was encouraged by the zeal of O’Malley and Gregoire.

“These governors have put their heart and soul into these laws and are serious about defending them at the ballot box,” he said. “That’s a factor the opponents don’t have.”

The opponents do have the Catholic Church leadership firmly on their side. The Minnesota Catholic Conference has already contributed at least $750,000 for the amendment campaign, and in Maryland and Washington, Catholic leaders also have pledged active campaigns.

In Maine, the diocese plans to expand an educational program to inform parishioners about the benefits of marriage between one man and one woman.

But unlike 2009, when the diocese contributed more than $500,000 and its public affairs director took a leave of absence to lead the effort to overturn the state law, the diocese plans no active role in the referendum campaign this time around, according to Bishop Richard Malone.