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 is an election-year subplot sure to stir heated emotions – even beyond North Carolina, Minnesota, Maryland, Maine, and Washington state. National advocacy groups will be deeply engaged, and advertising is likely to surface from each side.

“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 to 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 prevailed.

However, the most recent vote was in 2009. Gay-rights activists think 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 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. “The other side has remained very stale and stagnant.’’

A look at the 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 – similar to those approved in 29 other states – would prevent courts from empowering same-sex couples

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