Yesterday something historic happened. A sitting president and current candidate for the office stated his support for same-sex marriage. This is monumental because as little as ten years ago a president making such a statement was almost unthinkable.

It was, after all, only 16 years ago that the Democratic President Bill Clinton signed the Defense of Marriage Act, which codifies the federal non-recognition of same-sex marriage. The law easily won approval in Congress with broad bipartisan support. At the time Gallup found only 27% of Americans thought same-sex marriage should be valid while 68% of the country opposed it.

Same-sex marriage was once a fringe idea supported by only a tiny minority of the population and almost no politicians, yet in less than a generation things have completely changed. It now has the support of half the population and the de facto leader of one of the two major political parties.

Marriage equality is a fascinating example of how a country can rather quickly undergo a paradigm shift — an example of how, through activists, efforts to build public acceptance in the popular culture and demographic changes a fringe idea can become part of the political mainstream.

It is a particularly useful example for the marijuana legalization movement, which is experiencing an almost identical trend of rapidly growing support driven by a younger generation. As with marriage equality, less than 20 years ago marijuana legalization was an incredibly fringe issue with little popular or political support. In 1995, just 25% of Americans thought it should be legal, with 73% thinking it should be illegal.

Yet just 16 years later in 2011, Gallup found 50% thought marijuana should be legal and only 46% thought it should be illegal — putting popular support for marijuana legalization right on par with that for same-sex marriage

As with both marriage equality and marijuana legalization, much of the change in popular support is generational, thanks to a young, highly accepting generation replacing an older more conservative one. In 2011, 62% of adults under 30 thought marijuana should be legal but just 31% of those over 65 thought it should. An almost equal generational divide exists for marriage equality. That same year 68% of those under 30 supported it but only 33% of senior citizens thought that it should be allowed. For both issues, the nature of demographics should continue to push public support up for years to come.

The support of a sitting president for marriage equality is a powerful tipping point for the issue. It clearly signaled that this once highly fringe position is now firmly in the political mainstream. Looking at the similar polling and demographic trends for marijuana legalization, it might not be long before it experiences a similar symbolic victory publicly signaling its emergence as a mainstream political stance.