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Ms. Saad said the latest numbers don’t show a clear trend toward pro-life identification. “Since 2009, pro-choice has not hit 50 percent [support] and pro-life has not gone below 44 percent. And that’s completely different than the pattern prior to that,” she said.

However, it “remains to be seen whether the pro-life spike found this month proves temporary, as it did in 2009, or is sustained for some period.”

Asked about what might be influencing people to change their views, Ms. Saad said, “we don’t know why,” but that it’s possible certain events could have an effect, at least temporarily.

For instance, in the late 1990s, there was a sharp drop in pro-choice identification and rise in pro-life identification.

Those changes “corresponded to the debate over partial-birth abortion,” she said. Based on some polls, “we speculated that suddenly the definition of what it meant to be pro-choice or pro-life hinged on this issue — partial-birth abortion — which people were against. And that changed the framework for what those labels mean.”

Looking at today’s issues, Ms. Saad said, polling on the Obama administration’s battle with religious leaders over a health insurance mandate to cover all contraception products showed that the issue is “not a slam-dunk for either side.”

“But who knows what buttons are being pushed in people’s minds?” she said.

John Sides, a professor of political science at George Washington University, cautions against reading too much into the headline pro-life versus pro-choice totals, saying the labels can obscure as much as reveal popular attitudes about public policy and the often complex, personal choices about abortion.

“This ‘pro-life’ versus ‘pro-choice’ question obscures the true nature of American attitudes toward abortion. Support for the right to abortion depends strongly on the circumstances of the pregnancy,” Mr. Sides wrote Wednesday on the political science blog the Monkey Cage.

Other numbers from Wednesday’s Gallup poll:

• Fifty-one percent of Americans say abortion is “morally wrong,” while 38 percent say it is “morally acceptable.” These are both in line with previous polls.

• Democrats remain overwhelmingly pro-choice, with 58 percent identifying as supporters of abortion rights. However, the figure reflects a 10-point decline since 2011, when 68 percent of Democrats said they were pro-choice.

• Republicans registered a near-record high of 72 percent identification as pro-life, and a record low of 22 percent as pro-choice.

• Political independents typically have identified themselves as pro-choice, including 51 percent who chose this view in 2011. However, the new poll showed a reversal in positions, with 41 percent of independents saying they are pro-choice compared with 47 percent saying they are pro-life. This means that for only the second time, “pro-lifers now outnumber pro-choicers among this important swing group,” Gallup said.