David Blankenhorn, a traditional-marriage advocate and star witness in the Proposition 8 trial in California in 2010, shocked his allies with an Op-Ed article in The New York Times last June announcing that he was quitting the fight against same-sex marriage. “Instead of fighting gay marriage,” Mr. Blankenhorn wrote, “I’d like to help build new coalitions bringing together gays who want to strengthen marriage with straight people who want to do the same.”

He is about to find out how much support such a coalition can get.

On Thursday, Mr. Blankenhorn’s research group, the Institute for American Values in New York, plans to issue “A Call for a New Conversation on Marriage,” a tract renouncing the culture war that he was once part of, in favor of a different pro-marriage agenda. The proposed conversation will try to bring together gay men and lesbians who want to strengthen marriage with heterosexuals who want to do the same.

The document is signed by 74 well-known activists, writers and scholars, on the left and the right, including the conservative John Podhoretz, editor of Commentary magazine; John Corvino, a gay philosopher; Robert N. Bellah, a sociologist; Caitlin Flanagan, a social critic; and Glenn C. Loury, an economist — once conservative, now less so.

“While the nation’s attention is riveted by a debate about whether a small proportion of our fellow citizens (gays and lesbians) should be allowed to marry,” the statement reads, “marriage is rapidly dividing along class lines, splitting the country that it used to unite.”