Ironically (given the political divide) the driving force is the same behind the increase in those who are pro-life and the increase in those who are pro-SSM: raw broadening of empathy.
“If we must have an enemy at the head of Government, let it be one whom we can oppose, and for whom we are not responsible, who will not involve our party in the disgrace of his foolish and bad measures.”
- Alexander Hamilton. Spiritual father of #NeverTrump
Abortion | Gallup Historical Trends
This graph looks a lot different than those in connection with same sex marriage.
There hasn't been a huge amount of change with time when it comes to abortion. Most of the opposition/support on the abortion issue comes with specifically where the limitations should or should not be. The majority feels it should be legal under limited circumstances, while the other two options had low numbers for both, all abortion illegal or all abortion legal.
And there is very little generational difference. The biggest difference being in whether all abortions should be illegal, which still does not have a majority in that younger age group. What has the majority is abortion legal under certain circumstances.
Generational Differences on Abortion Narrow
Otherwise all groups have been changing with time, trending up and down between decades. We may (and likely will) see another change in the future where support with all age groups for abortions (at least limited) grows.
The graph for same sex marriage looks a whole lot like the graph for interracial marriage, only over a lot fewer years.
"A woman is like a teabag, you never know how strong she is until she gets in hot water." - Eleanor Roosevelt
Keep your religion out of other people's marriages.