- Advocates for same-sex marriage set an ambitious timetable for a nationwide victory, laying out a plan for a rapid series of campaigns to win over additional states to achieve that goal.

"Within five years, we will bring marriage equality to all 50 states," Chad Griffin, president of the Human Rights Campaign, pledged to supporters outside the Supreme Court shortly after the court's decisions Wednesday.

The rulings will bring to 13 the number of states, along with the District of Columbia, allowing gay marriage; together they account for about a third of the country's population. But among the remaining states are many deeply conservative ones that probably will resist the nationwide shift toward acceptance of same-sex relationships.

Over the next 18 months, gay rights groups plan legislative or ballot campaigns in at least five states — Illinois, New Jersey, Nevada, Oregon and Hawaii. Same-sex marriage is also before the courts in New Mexico, where state law is unclear.

Seeing a return to high court

Ultimately, however, advocates of same-sex marriage believe that they will have to argue the issue once again before the high court.

"The strategy has always been to secure a critical mass of states and a critical mass of public opinion in favor of marriage equality, and with that climate engage the Supreme Court," said Evan Wolfson, president of Freedom to Marry and one of the leading strategists for the same-sex marriage movement.

For gay rights advocates, the key to success is establishing equal marriage rights for same-sex couples as the norm in the eyes of the American public. This week's rulings indicate that a majority of the justices sympathize with the legal arguments in favor of same-sex marriage. The more widely public opinion and the law come to accept same-sex unions, the more willing a future Supreme Court will be to adopt those arguments and sweep away the remaining barriers, they believe.

Opponents largely share that assessment. As a result, the two sides probably will battle on a number of fronts.

Gay rights groups also have begun to lobby the Obama administration to quickly clarify the rules for federal programs to ensure that the maximum number of same-sex couples qualify for federal benefits.

Opponents of same-sex marriage, who had backed national laws such as the Defense of Marriage Act, hope to slow the movement by insisting that the issue now be decided on a state-by-state basis.

Reconciling federal programs

One early set of skirmishes will center on the hundreds of federal programs that Wednesday's rulings have opened to same-sex couples married in states where gay marriage is recognized.

Currently, some federal programs recognize marriages based on the law in the state in which a couple lives. Others look to the law in the state in which the marriage was celebrated.

As a result, a gay couple who marry legally in California, then retire in Arizona, for example, might keep some federal benefits but lose others.

On Thursday, President Obama signaled that his administration would extend federal benefits to gay couples living in states that don't recognize their marriages, saying the government should define marriage based on where couples are wed.

"It's my personal belief — but I'm speaking now as a president as opposed to as a lawyer — that if you've been married in Massachusetts and you move someplace else, you're still married, and that under federal law you should be able to obtain the benefits of any lawfully married couple," Obama told reporters in Dakar, Senegal.

Homeland Security Secretary Janet Napolitano said her department planned to issue guidance soon. Defense Secretary Chuck Hagel also announced that the Pentagon would move quickly to extend benefits to same-sex couples.

Some programs, such as the immigration system, already use the standard that gay rights groups prefer. As a result, gay rights groups say they believe the U.S. Citizenship and Immigration Services will now be able to award lawful permanent residency to same-sex spouses of American citizens regardless of what state the American partner lives in.

"This decision is literally life changing for more than 36,000 families in the U.S. and many more living in exile abroad," said Rachel B. Tiven, executive director of Immigration Equality, a legal aid and advocacy organization based in New York that pushes for federal immigration benefits for married same-sex couples.

The Washington Post contributed to this report.