The issue is important because congressional passage of healthcare reform could hinge on language over abortion funding, Gallup said in a report released Monday.
Since 1975, two years after the Supreme Court approved constitutional protection for abortion, Gallup has polled U.S. residents annually on whether they believe abortion should be legal or illegal.
Overall, Republicans and Democrats held similar views on abortion in the 1970s and 1980s, saying abortion should be legal "only under certain circumstances," Gallup said. Since 1990, however, Republicans polled by Gallup increasingly have grown more conservative about the issue while Democrats have become more liberal.
By 2009, more Republicans -- by a 21-point margin -- said abortion should be illegal "in all circumstances," and more Democrats - by a 19-point margin -- said abortion should be legal "under any circumstances."
The Gallup report, which had a margin of error of 3 percentage points, was based on surveys from 1975 through 2009 with about 1,000 adults interviewed annually.