Interestingly on the UCSD campus where I go to school.
A 1991 poll showed 77% say "yes" to whether abortion should be legal or not. By 2002 this number had dropped to 70%.
I think the main reasons for this swing if anything were the widespread availability of contraceptives, condoms and the coming of the pill.
Also interestingly during this time the amount of people who identified as "conservative" actually fell from 19% to 15%. While those who identify as far left increased from 2% to 3% and those who identify as liberal increased from 33% to 36%. The amount that identified as "moderate" stayed static at 46%
...you also got to remember that 1991 was a peak of pro-choice activism in this country and 2002 was a peak of religious right influence in this country.
Even than, self-described liberals increased by 4% and self-described conservatives decreased by 4%.
