The findings, obtained in an annual survey on values and beliefs conducted May 7-10, marked a significant shift from a year ago. A year ago, 50 percent said they were pro-choice and 44 percent pro-life - in the new poll, 42 percent said they were pro-choice.
The U.S. Supreme Court legalized abortion across America with its 1973 Roe vs. Wade verdict, but the practice remains controversial and some states have tried to restrict it.
The new survey showed that Americans remained deeply divided on the legality of abortion - with 23 percent saying it should be illegal in all circumstances, 22 percent saying it should be legal under any circumstances, and 53 percent saying it should be legal only under certain circumstances.
The findings echoed a recent national survey by the Pew Research Center, which reported a sharp decline since last August in those saying abortion should be legal in all or most cases - from 54 percent to 46 percent.
Taken together, the two polls have elated anti-abortion activists, who had been stung by the November election results that placed President Barack Obama and other abortion-rights supporters in power in Washington.
"Ironically, Obama's radical abortion policies and nominees may have helped make America more pro-life," said Wendy Wright, president of the conservative advocacy group Concerned Women for America.
The Rev. Richard Land, president of the Southern Baptist Convention's Ethics & Religious Liberty Commission, said the poll findings demonstrate that the anti-abortion cause "is a vibrant, growing, youthful movement."
"We are winning the battle for hearts and minds in our culture on the life issue," he said.
The president of a leading abortion-rights group, Nancy Keenan of NARAL Pro-Choice America, said the Gallup findings "do not square with the voting patterns in the last two elections cycles."
"It would be a mistake for anti-choice groups to interpret this one poll as a signal that Americans want even more interference from politicians in their personal, private decisions, including a woman's right to choose safe, legal abortion," Keenan said.
Another abortion-rights leader, Planned Parenthood Federation of America president Cecile Richards, questioned the terminology in the Gallup questions.
"The terms pro-choice and pro-life no longer define the parameters of the debate, witnessed by the fact that in the Gallup Poll, a majority of people say they are both pro-life and that abortion should be legal," Richards said.
She added that most Americans share Obama's stated goal of reducing the number of unintended pregnancies.
Planned Parenthood also noted that another recent national survey, a CNN/Opinion Research Corp., poll in April, reported 49 percent of respondents identifying as pro-choice and 45 percent as pro-life.
The Gallup poll's release came just ahead of Obama's scheduled commencement speech Sunday at the University of Notre Dame, where he also is to receive an honorary degree. Those plans by the Roman Catholic university have sparked a wave of protests by anti-abortion activists, who contend Notre Dame should not honor a such a prominent supporter of abortion rights.
Gallup said its new poll showed an increase in the pro-life position across Christian religious affiliations, including an eight-point gain among Protestants and a seven-point gain among Catholics. It also reported a 10-point shift toward the pro-life category among Republicans but said there was no significant change among Democrats.
In the new poll, men identify as pro-life, 54 percent to 39 percent, while women also tilt pro-life 49 percent to 44 percent. A year ago, Gallup found more women calling themselves pro-choice than pro-life, by 50 percent to 43 percent, while men were more closely divided: 49 percent pro-choice, 46 percent pro-life.
"It is possible that, through his abortion policies, Obama has pushed the public's understanding of what it means to be 'pro-choice' slightly to the left, politically," according to the Gallup analysis. "While Democrats may support that, as they generally support everything Obama is doing as president, it may be driving others in the opposite direction."
The Gallup survey was based on telephone interviews with 1,015 adults nationwide. Its margin of sampling error was plus or minus 3 percentage points.