In the aftermath of Friday’s Iowa Supreme Court decision, Nate Silver, who earned his stripes as a political prognosticator during the 2008 presidential election, predicts that nationwide it will become harder and harder to pass bans on same-sex marriage over time.

At his blog, FiveThirtyEight.com, Silver examined trends in the 30 states that have voted on same-sex marriage bans and found they are becoming more difficult to pass every year.

Marriage bans, however, are losing ground at a rate of slightly less than 2 points per year. So, for example, we’d project that a state in which a marriage ban passed with 60 percent of the vote last year would only have 58 percent of its voters approve the ban this year. …

… So what does this mean for Iowa? The state has roughly average levels of religiosity, including a fair number of white evangelicals, and the model predicts that if Iowans voted on a marriage ban today, it would pass with 56.0 percent of the vote. By 2012, however, the model projects a toss-up: 50.4 percent of Iowans voting to approve the ban, and 49.6 percent opposed. In 2013 and all subsequent years, the model thinks the marriage ban would fail.

The earliest Iowans would get a chance to vote on a constitutional amendment banning same-sex marriage would be late 2011.

Silver’s logic matches well with a recent poll conducted by the University of Iowa which found 60 percent of Iowans under age 30 support same-sex marriage, and three-fourths of Iowans under 30 favor some formal recognition of same-sex relationships. David Redlawsk, an associate professor of political science at the University of Iowa who oversaw the poll, said the figures show passion for banning same-sex marriage diminishing over time.

Silver predicts that by 2012, almost half of the 50 states would vote against a marriage ban, including several states that had previously voted to ban it.