Nate Silver, who’s Web site FiveThirtyEight.com became mandatory reading during this year’s election, has an interesting take on the success of California’s Proposition 8, a ballot initiative to amend the state’s Constitution to restrict the definition of marriage to a union between a man and a woman (the measure passed, but is being challenged by several lawsuits).

Silver points out that if those over the age of 65 did not vote, the measure would have failed.

At the end of the day, Prop 8’s passage was more a generational matter than a racial one. If nobody over the age of 65 had voted, Prop 8 would have failed by a point or two. It appears that the generational splits may be larger within minority communities than among whites, although the data on this is sketchy.

The good news for supporters of marriage equity is that — and there’s no polite way to put this — the older voters aren’t going to be around for all that much longer, and they’ll gradually be cycled out and replaced by younger voters who grew up in a more tolerant era. Everyone knew going in that Prop 8 was going to be a photo finish — California might be just progressive enough and 2008 might be just soon enough for the voters to affirm marriage equity. Or, it might fall just short, which is what happened. But two or four or six or eight years from now, it will get across the finish line.

This logic has implications here in Iowa, where the state Supreme Court is scheduled to hear arguments in the legal challenge to the state’s same-sex marriage law on Dec. 9. The topic of amending Iowa’s Constitution to ban same-sex marriage has been talked about for a number of years, but if the Court rules in favor of the plaintiffs, Republican outrage could be hard to ignore. In Iowa, amendments must be passed by the legislature in two separate sessions before being put on the ballot for a vote.

Unlike California, Iowa’s population is graying. Data from the U.S. Census Bureau shows that the Hawkeye State has a larger percentage residents older than 65 than does California, and that percentage is growing. By 2030, the number of Iowans 65 and older and those younger than 18 will be roughly the same.

If you go by Silver’s thesis, that an older population means a greater likelihood of a law similar to California’s Prop 8 passing, Iowa looks like fertile ground for those waging a battle against same-sex marriage to set up shop.

(h/t to John Deeth)