This graph shows what the generation of Americans born in the 1980s might look like if Roe v. Wade had never happened. This generation has already been termed the "baby boom echo" because their parents were mostly born at the height of the baby boom in the late '50s and early '60s, and thus by sheer numbers made their children's generation pretty large even though the birth rate was a lot lower in the "echo" years. But if we factor in the roughly 13 million abortions that occurred in the 1980s, we see that the '80s would have witnessed a boom in births that actually significantly surpassed the size of their parents' boom (at least in absolute size, though perhaps not in percentage of the population). And that makes sense, because in the pre-Roe era, each generation of Americans tended to replace itself with a larger cohort of offspring. So my thought experiment is: what would our voting population look like with all those extra twentysomethings in it? See below the fold for the answer...
To begin with, I adjusted for the racial skew in who got abortions (African Americans are overrepresented in the abortion numbers by at least a factor of 3), and assumed* that in other respects, this group of people would have had rates of participation and candidate preference similar to those in this age group who were actually born.
My back-of-the envelope calculations gave me roughly 4.7 million voters to add to Obama's total, and 1.7 million to add to McCain's. This would have changed Obama's 2008 7.2 point victory to a 9.2 point margin. Overkill in his case, but a bump that size (or even one half that size) could come in handy in a nail-biter type election. And that doesn't even factor in the fact that the alternate reality we're talking about here would most likely have more congressional districts (and thus more electoral votes) concentrated in blue states, and fewer of both in red states.
None of this, I should hasten to point out, means that Roe v. Wade was a bad development for America. To the contrary: safe and legal abortion is an important right women must have. But it is pretty ironic that Republicans have been fighting so hard for decades against a decision that actually gave their party a lifeline it has sorely needed (and may yet again rely on in the future) to eke out narrow electoral victories. "Be careful what you wish for", and all that.
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*Admittedly, this is ultimately unknowable. But I can think of arguments for why, even after correcting for racial makeup, this population might have been more likely to lean Democratic than Millennials overall. After all, they almost had to be conceived by women who were less likely than the population as a whole to be socially conservative. (Yes, there are plenty of hypocrites out there; but I still think that on average this will tend to hold true.) I also didn't include any effect of voters who would have been born in the 1970s, even though they also skewed toward Obama albeit by a smaller margin (but then there is still the factor of race there as well). I can't on the other hand think of any arguments for why this group would have been likely to skew more toward Republicans than their general demographic profile would predict them to--but that doesn't mean there aren't any and I'd be glad to hear them. For now, I think assuming they will vote the same as the rest of their cohort, after you account for race, is likely to be a pretty safe bet.