A proposed constitutional amendment against gay marriage in California — already lagging among likely voters — continues to lose support, according to a new Field Poll released today, while the percentage of those who say they oppose the ban appears to be on the rise.

Fifty-five percent of voters indicated they would say no to the proposed amendment, which would make same-sex marriage illegal in California. That is up from 51 percent opposing the measure, Proposition 8, in a previous poll conducted in July. In the sampling of more than 800 likely voters, only 38 percent said they would support the ban, down from 42 percent nearly three months ago.

“Initiatives that are trailing, either at the initial measurement or in subsequent measures, rarely pass,” said Mark DiCamillo, director of the Field Poll, also noting that opinions on issues like same-sex marriage tend not to fluctuate dramatically. “History is working against passage.”

But that trend is hardly surprising, a spokeswoman for the “Yes on 8” campaign said, especially after the state won approval last month to change the ballot language, specifically mentioning a ban on same-sex marriage.

“That was certainly something we expected to see,” Jennifer Kerns said, adding that, at the same time, “historically, the Field Poll has underestimated support for traditional marriage.”

The poll also sought to examine what effect that new ballot language had on voters’ views. It found that opposition to the ban increased slightly when voters were shown the new language instead of the original. Those changes were unsuccessfully challenged in court by supporters of the proposition, who argued it was misleading.

To reach that finding, voters in the study were randomly divided into two groups, with one receiving the original ballot language to appear on the ballot. The other group received the new language, which will appear on the Nov. 4 ballot.

While support for the measure remained static at 38 percent, opposition increased from 52 percent to 55 percent.

“That has to do with the description,” DiCamillo said. “Eliminating an individual right, no matter what that right might be, is not as popular as the alternative.”

When broken down across party affiliation, religion or geography, today’s Field Poll revealed a handful of other, subtle changes when compared with July’s poll, which marked the first independent examination of the issue following the California Supreme Court’s decision June 4 to remove the last hurdles to same-sex marriages.

More voters in inland counties now appear opposed to the measure than in favor, 48 percent to 44 percent. That’s a significant shift from July, when 40 percent said they opposed the ban with 54 percent in favor. The new poll also showed slightly softening support among Protestants and Catholics and among Democrats and Republicans alike.

Despite the widening margin of those who say they oppose a constitutional ban, the groups fighting to defeat Proposition 8 “won’t let up,” said Molly McKay of Marriage Equality USA, part of the “No on 8” coalition.

Still, she said, “I think most fair-minded people can’t imagine changing the California Constitution to take away civil rights. That’s just not what California is all about.”

But Kerns again cautioned against reading too much into the new Field Poll results, explaining that other polls this summer have shown support for the ban leading and that the campaign’s own internal polling shows approval of the measure at a dead heat.

“The most important numbers are the ones that happen on Election Day,” she said. “But no doubt about it, we expect it to be very close all the way through.”

Contact Denis C. Theriault at dtheriault@mercurynews.com or (408) 920-5035.