2014 was a pivotal year for same-sex marriage

A year ago, only about a third of Americans lived in states that permitted same-sex marriage. Today, nearly 65 percent of Americans do, making 2014 perhaps the biggest turning point in the history of same-sex marriage in the United States.

The change wasn’t driven by a bevy of new laws or a big Supreme Court decision. Instead, it was a slow-burning sequence of consistent lower court rulings — combined with the Supreme Court’s decision not to intervene.

For thousands of same-sex couples, the effect has been transformative. And it may end up reshaping the political landscape as well, because same-sex marriage doesn’t break down cleanly along party lines. Three of every five young Republicans now say they support same-sex marriage.

Where is same-sex marriage legal?

Massachusetts was the first state to allow same-sex marriage. In the 10 years since, same-sex marriage has spread to 34 other states, including 18 new states just this year. States that continue to ban the practice tend to be concentrated in the South and the Midwest.

Source: Pew Research Center.

What drove this year’s expansion of same-sex marriage?

In a streak of victories in federal courts — and especially in the appellate courts that make law for whole regions of the country — various statewide bans on same-sex marriage were declared unconstitutional.

At first, many of those rulings were put on hold, which is not uncommon. Sometimes, lower courts want to hit the pause button and let the Supreme Court weigh in before implementing big social changes. But in October, the Supreme Court decided not to intervene — they didn’t say why, but it may be because there was no dispute for them to resolve. The pro-marriage side had won every case.

Once the Supreme Court declined to step in, those lower-court rulings became law and same-sex marriage expanded.

Will the Supreme Court eventually intervene?

Shortly after the Supreme Court decided not to review those earlier cases, a disagreement finally occurred. The sixth circuit — which covers Michigan, Ohio, Kentucky, and Tennessee — became the first appeals court to uphold a state ban on same-sex marriage.

A split like this in the lower court greatly increases the likelihood that the Supreme Court will decide to rule on the constitutionality of same-sex marriage — possibly as early as this coming spring. While it’s impossible to say with any certainty how the court will rule, same-sex marriage advocates are optimistic. So much so that they’re actually pressing the court to intervene now (not least of all because a single new justice could upend the current balance).

What about public opinion?

Public opinion has been shifting in favor of same-sex marriage since about 2009, and this year was no exception. The number of Americans supporting same-sex marriage crossed 50 percent last year and reached 54 percent this year.

It’s not just Democrats who support gay marriage, it’s also younger Republicans.

What happens next?

As pivotal as 2014 has been in the history of gay marriage, 2015 may be definitive. If the Supreme Court rules in favor of gay marriage, the United States would join the growing ranks of countries that have embraced it. Alternatively, a Supreme Court ruling against gay marriage would force the issue back to the states, potentially leading to a dramatic retrenchment in the number of same sex couples who are able to marry.