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 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.