For the past year, same-sex marriage proponents have been on a winning streak, including on Tuesday when the Illinois legislature approved gay weddings starting in June. While Hawaii is expected within a week to become the 16th state sanctioning same-sex marriage, activists will face a tougher time expanding the map after that point.

Any future change in state marriage laws rests almost entirely on either the reversal of  gay wedding bans through ballot initiatives or court challenges, both of which represent a steeper climb. The one exception to this is in New Mexico, where the state's constitution is silent on the question: its Supreme Court heard arguments on a case involving six couples last month and is expected to rule on the question within a matter of months.

"We're going to be entering an era where most of the legislative fights are over, and we're going to have to undo constitutional amendments, and do that at the ballot box," said Marc Solomon, national campaign director of the group Freedom to Marry.

That's terrain that has traditionally favored defenders of traditional marriage. Twenty-nine states now have voter-approved constitutional bans on gay marriage (since California's was overturned in June); one state, Minnesota, defeated a proposed ban; and three states -- Maine, Maryland and Washington -- legalized same-sex weddings through a statewide vote.

National Organization for Marriage president Brian Brown said that even Illinois and Hawaii -- which have large Democratic majorities in their legislatures -- have been difficult lifts for the other side. "Now, if they want to redefine marriage, they’re going to be on turf that's much more difficult."

Fred Sainz, vice president for communications and marketing at the Human Rights Campaign, agrees. "We are quickly running out of states where this is an easy lift," he said.

The two biggest battles next year are likely to be Oregon, where gay marriage proponents hope to overturn the state's ban, and Indiana, where traditional marriage advocates hope to pass a constitutional amendment banning gay marriage for the second time in a row. A public education campaign supporting same-sex marriage is already underway in Arizona, and while some local activists have been pressing for a ballot initiative there in 2014, it is more likely that national groups will throw their weight behind a 2016 push.

Three years from now, several states could see ballot initiatives to legalize gay weddings, including Colorado, Ohio and Michigan.

In the meantime, there are nearly 40 challenges percolating through the court system, in states such as Virginia, Pennsylvania, Michigan and Texas.

"The marriage movement has never put its eggs in one basket," Sainz said, adding that some of these cases will inevitably make it to the nation's highest court. "No one is under any illusions that marriage is going to come to all 50 states through any other venue than the United States Supreme Court."

Brown said he was worried about this very prospect, since gay marriage advocates have often found judges receptive to their argument.

"What they’re really hoping for is they can avoid the ballot initiatives as much as possible. That’s really the end game," he said. "Are we concerned about the use of the courts to legislate and reinterpret the law? Of course we are."

In the meantime, however, groups such as HRC and Freedom to Marry are trying to ensure that a majority of Americans live in states where same-sex marriage is legal by the time the Supreme Court considers this question once more.

What does the math look like, based on 2010 Census data? Once Illinois Gov. Pat Quinn (D) signs the recently passed bill, 37.2 percent of Americans will be living in such states; if Hawaii Gov. Neil Abercrombie (D) does the same, it will be 37.7 percent. If it becomes legal in New Mexico, the number inches up to 38.4 percent; Oregon would bring it to 38.6 percent. If Arizona, Colorado, Michigan, Ohio and Nevada all approved of same-sex marriage, 51.5 percent of all Americans would be living in states where gay and lesbian residents could marry.

In other words, the fight continues. And it will for at least a few more years.