The main group opposed to the amendment raised $7.8 million by the end of September. The leading group supporting it raised about $2 million, nearly half of that from Catholic dioceses and affiliated organizations.
A Minneapolis Star Tribune poll last month found 49 percent of likely voters supporting the amendment and 47 percent opposing it, within the poll's margin of error.
In all four states, TV ads airing against same-sex marriage are the brainchild of political strategist Frank Schubert, whose ads were credited with a key role in California's passage of the Proposition 8 ban on gay marriage in 2008.
Schubert's strategy is to laud heterosexual marriage as a timeless institution that should not be "redefined" and to warn that legalization of same-sex marriage can impinge on the rights of those who oppose it. He says such ads offer "solid lines of argument," while his gay-rights rivals assail them as deceptive scare-mongering.
To Schubert, the four-state showdown is "a big deal" — in part because the U.S. Supreme Court is expected to take up the question of same-sex marriage soon.
"We don't want to lose anywhere," Schubert said. "If one state does go the wrong way, we'll argue that this is just one of out of 36 ... But we'd rather be arguing we've won every time it has gone before voters."
Associated Press writers Rachel La Corte in Olympia, Wash., Patrick Condon in Minneapolis, Clarke Canfield in Portland, Maine, and Brian Witte in Annapolis, Md., contributed to this report.
David Crary can be followed on Twitter at http://twitter.com/CraryAP
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