For most of the last two decades, a clear majority of Americans has supported the right to abortion. A new poll, though, suggests that support for abortion may have declined, with the public almost evenly divided over the issue.

The apparent shift, which contradicts some other recent polls, appears in a poll by the Pew Research Center released on Thursday. A 2008 poll by Pew researchers had found that those in favor of keeping abortion legal outnumbered opponents, 54 percent to 40 percent. In the new Pew poll, the gap has narrowed: 47 percent of those surveyed said abortion should be legal in all or most cases, and 45 percent said it should be illegal in all or most cases—a difference within the poll's margin of sampling error.

The new survey did not find the reasons for the shift in opinion. But Pew researchers pointed out that the shift has occurred since the election of President Barack Obama, a Democrat who supports abortion rights but has often spoken about the need to reduce the number of abortions. New York Times/CBS News polls, as well as others, found no such shift after Bill Clinton—also a Democrat who supports abortion rights—took office. Pew has no comparable data on the issue dating back that far. Polls conducted by some other organizations within the last few months have found opinion on abortion to be more stable. In an ABC News/Washington Post poll in June, for example, 55 percent of respondents said abortion should be legal in all or most cases, while 43 percent said it should be illegal in all or most cases—close to the average in their polls since 1995.

Times/CBS News polls have also found essentially no change in Americans' overall views of abortion. In a Times/CBS News poll in June (which asked a differently worded question than the Pew or ABC/Post polls did), 36 percent said abortion should be generally available; 41 percent said it should be available but under stricter limits than it is now; and 21 percent said it should not be permitted—close to the average in New York Times or CBS News polls dating back 20 years.

The inconsistency between polls makes it hard to draw a firm conclusion about whether attitudes are shifting. And it underscores how sensitive poll results are to questions about abortion.

The change found in the new Pew poll happened among many demographic groups, to varying degrees—among women and men; Republicans, Democrats and independents; Protestants, Catholics and Jews; whites and Hispanics. There was no change among blacks and people who have no religious affiliation.

“;The size of the shift is modest, but the consistency with which we see it occurring and the implications it has for the overall dynamics of the debate make it significant,”; said Gregory Smith, a senior researcher at the Pew Forum on Religion and Public Life.

The poll is based on telephone interviews among a nationwide sample of 4,013 adults. Interviews were conducted in two waves, the first from Aug. 11 through 17 and the second from Aug. 20 through 27. The margin of sampling error is plus or minus two percentage points for all respondents, and larger for subgroups.

The abortion issue, which faded as a factor in last year's presidential elections, has resurfaced in the debate over health care overhaul. Abortion opponents are threatening to derail any plan that allows government money to be used for abortions.

Asked how Obama will handle the abortion issue, about 3 in 10 respondents in the Pew poll said he would handle it about right, and 2 in 10 (including more than half of the conservative Republicans in the poll) worried that he would go too far in supporting abortion rights. Four in 10 of those polled were unaware of Obama's stance on abortion.

The Pew poll suggests that supporters of legalized abortion may have grown complacent. Only 8 percent of those who identified themselves as liberal Democrats said abortion was a critical issue facing the country today, compared with 26 percent of conservative Republicans.

But among all respondents, concern about the issue has dropped considerably. Only 15 percent of respondents said abortion was a critical issue, compared with 28 percent in another survey conducted in 2006.