The poll by the Pew Research Center found that 63 percent of Americans believe that Roe v. Wade should not be completely overturned, compared to 29 percent who believe it should be. These opinions have changed little from surveys conducted in 2003 and 1992, Pew reported.

Michael Dimock, director of the Pew Research Center for the People and the Press, said it is uncommon to see so little change in attitudes on a controversial issue.

"They really haven't changed a lot over the years which is kind of interesting because a lot of other social issues have changed a lot, gay marriage being the most notable example," said Dimock.

He noted that opinions on issues such as gay marriage sometimes have a sharp generational divide, with younger people more likely to favor it, so national feelings change over time.

But the abortion issue shows only modest generational differences, and no gender gap.

Those most likely to favor upholding Roe v. Wade at 69 percent are the "baby boomers" aged 50-64, who were children or young adults when the case was decided on January 22, 1973. This group was followed by those 18-29 years old, who favored upholding the decision by 68 percent.

NARAL Pro-Choice America, a national abortion rights group, said the data confirms what NARAL has found in its own research.

"This poll is a reminder that the public clearly agrees, and has done so for decades," said Tarek Rizek, communications director for NARAL.

Joseph Scheidler, prominent abortion opponent and national director of the Pro-Life Action League, said opinions about abortion have changed since 1973 because of advances like ultrasound, which allow a better understanding of fetal life.

"I don't worry much about these polls..." said Scheidler. "I think a majority of Americans prefer to be called pro-life." He said knocking down Roe v. Wade - which would return the issue to the states - is not as important as educating people on "the great evil" of abortion.

The Pew poll also found that 53 percent of the U.S. public say the issue of abortion is not that important compared to other issues - the first time that number has been over 50 percent. Dimock said this may reflect Americans' current preoccupation with issues such as the national debt and gun control.

There are still wide religious differences over whether to overturn Roe v. Wade and the morality of abortion, the poll found. White evangelical Protestants are the only religious group in which a majority - 54 percent - favors overturning the decision.

Large percentages of white mainline Protestants (76 percent), black Protestants (65 percent) and white Catholics (63 percent) say the ruling should not be overturned.

U.S. Roman Catholic Bishops called for nine days of prayer and penance starting Saturday to mark the Roe v. Wade anniversary. In their press release, the bishops asked for prayers for "healing and conversion" for elected officials who support abortion and for all people whose lives have been affected by it.

The Pew poll also shows that 47 percent of Americans say they believe it is morally wrong to have an abortion. These opinions have changed only modestly in recent years.

Younger people are less likely to know what Roe v. Wade was about. While most respondents over 30 knew Roe v. Wade dealt with abortion, only 44 percent of those under 30 knew this, the poll found. The question over whether the decision should be overturned was asked after it was defined to respondents.

The poll was based on interviews with a national sample of 1,502 adults, aged 18 or over, with a margin of error of plus or minus 2.9 percentage points.

Full results of the poll can be seen here: here

(Reporting By Mary Wisniewskil; Editing by Paul Thomasch, Claudia Parsons and Andrew Hay)