A new poll on Wednesday revealed that support for same-sex marriage is growing among Americans.

Forty-two percent favor allowing gays and lesbians to marry legally, up from 37 percent last year, according to the Pew Research Center's latest survey.

Meanwhile, for the first time in 15 years, fewer than half oppose same-sex marriage. Forty-eight percent of Americans are now against same-sex couples marrying, a dip from 54 percent the previous year.

Notably, pluralities of white mainline Protestants and white Catholics now favor allowing same-sex marriage – another first for the Pew Research Center.

According to the poll, white mainline Protestants are more likely to support gay marriage (49 percent) than oppose it (38 percent). In the past two years, 49 percent had expressed opposition. Among those who attend church at least once a week, 35 percent favor gays and lesbians marrying.

A similar shift is seen among Catholics. While opinion was more evenly divided over the past two years, today 49 percent favor gay marriage and 41 percent oppose.

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White evangelical Protestants, meanwhile, remain overwhelmingly opposed to gay marriage. Seventy-four percent are against it and 20 percent favor it. A majority of black Protestants also continue to oppose gay marriage.

Evangelicals are also more likely than any other religious group to consider same-sex marriage as very important to their vote in the upcoming midterm elections.

Support overall for gays in the military is high, the poll also showed.

Sixty percent of Americans favor allowing gays and lesbians to serve openly in the military while only 30 percent oppose it. Protestants are also more likely to support gays serving openly in the military (52 percent) than oppose it (37 percent). Evangelicals, meanwhile, are more likely to stand against it (47 percent) than favor it (43 percent).

Results for the 2010 surveys are based two polls conducted over the past few months. Interviews were conducted with more than 6,000 adults.