WASHINGTON — Advocates for same-sex marriage set an ambitious timetable Wednesday 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 massed on the sidewalk 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 -- aimed at lengthening the list on their side. Same-sex marriage is also before the courts in New Mexico, where the state law is unclear.
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.
Gay right 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.
"In the U.S., marriage has traditionally been defined by state law, and I believe each state, acting through their elected representatives or the ballot, should decide their own definition of marriage," Sen. Marco Rubio (R-Fla.), who is widely seen as a potential GOP presidential nominee in the next election, said in a statement.
"These types of disagreements should be settled through the democratic process, as the founders intended, not through litigation and court pronouncements."
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. Wednesday'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.
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.
As gay rights groups push the administration to ensure that those benefits are available to the greatest extent possible, opponents of same-sex marriage, including some Republican leaders in Congress, already have pushed back. They portray the effort as an attempt to federalize same-sex marriage rights.
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.
Leaders of gay rights organizations talked with Atty. Gen. Eric H. Holder Jr. and other administration officials Wednesday to press their case that all programs should base their decisions on the law of the state in which the marriage was licensed, thereby eliminating the risk that a couple might lose benefits by moving.
Administration officials say they are studying that issue, hoping to avoid what one referred to as "unforced errors" that might prompt further lawsuits over benefits.
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.
Homeland Security Secretary Janet Napolitano said her department planned to issue guidance to immigration officials soon.
Defense Secretary Chuck Hagel also announced that the Pentagon would move quickly to extend benefits to same-sex couples.
Brian Bennett, Melanie Mason and Michael A. Memoli in the Washington bureau contributed to this report.