NEW YORK — The partisan gap over same-sex marriage continues to widen, with 65 percent of Democrats now supporting it compared to 24 percent of Republicans, according to poll released Tuesday by the Pew Research Center.

The poll found an increase in support among Democrats since President Barack Obama announced in May that he favors same-sex marriage. In April, a Pew poll gauged support among Democrats at 59 percent.

The latest poll, conducted jointly by the Pew’s Forum on Religion and Public Life and its Center for the People and the Press, was released a day after Democratic Party leaders said they intended to add support for gay marriage to the party platform at the Democratic National Convention in Charlotte, N.C., in early September.

At the time of the last convention, in 2008, 50 percent of Democrats favored allowing gays and lesbians to marry legally, while 42 percent were opposed. In the new poll, only 29 percent of Democrats were opposed.

According to Pew, support for gay marriage also has increased among independents. In the new poll, 51 percent of independents favor it, and 40 percent are opposed. In 2008, 44 percent of independents backed gay marriage, 45 percent were against it.

Advocates of gay marriage say the trends highlighted by Pew and other pollsters suggest that support for it by Obama and his party will be an asset in the Nov. 6 election. Foes of gay marriage disagree, noting that a majority of voters in several swing states have supported amendments banning gay marriage, including Ohio, Florida, Virginia and North Carolina, where the ban prevailed in the May 8 primary.

“There are many Democratic members of Congress, and officeholders further down the ticket, who live in states and districts where it will be a serious disadvantage to be identified with ‘the gay marriage party,'” said Peter Sprigg of the conservative Family Research Council.

Gay marriage will be on the ballot in four states on Nov 6. Voters in Maryland, Washington state and Maine will have a chance to join six other states in legalizing same-sex marriage, while Minnesotans will be voting on a ban-gay-marriage amendment.

Overall, Americans remain closely divided on same-sex marriage. The new Pew poll found 48 percent in favor and 44 percent opposed, virtually unchanged from a survey in April — before to Obama’s announcement.

Among blacks, 51 percent oppose gay marriage and 40 percent favor it, according to Pew — about the same as in April. Longer term, blacks have become much more supportive; in 2008 only 26 percent supported gay marriage.

The new poll was conducted by telephone June 28-July 9 among a random national sample of 2,973 adults, including 774 self-identified Republicans, 995 Democrats and 1,037 independents.

Results among all adults have a margin of sampling error of plus or minus 2.1 percentage points, including 4.1 percent for Republicans, 3.6 percent for Democrats and 3.5 percent for independents.