WASHINGTON — If a U.S. Supreme Court decision legalizing gay marriage looks inevitable, perhaps it is.

Since a pivotal high court ruling 13 months ago, gay-marriage advocates have tallied more than two dozen lower-court victories without a single defeat. With the July 28 decision in Virginia, two federal appeals courts have now backed same-sex marriage. Courts have consistently read last year’s ruling, U.S. v. Windsor, as undercutting any rationale for state bans.

The lower-court rulings have created a sense of legal momentum with few if any precedents in the nation’s history. And with the issue moving toward the Supreme Court, the question is becoming whether, in the face of such a legal rout, the justices can possibly disagree.

“I can’t think of any Supreme Court decision in history that has ever created so rapid and broad a lower-court groundswell in a single direction as Windsor,” said Laurence Tribe, a constitutional law professor at Harvard Law School. “Nor can I think of any historical examples in which lower courts have so overwhelmingly and universally read a Supreme Court decision one way, only to have the court say ‘Never mind, you’ve all gotten it wrong.’ “

MOST DEFINITIVE RULING

The 5-4 Windsor ruling in June 2013 is the high court’s most definitive take on the constitutional rights of gay couples. Striking down a 1996 U.S. law that denied federal benefits to legally married same-sex spouses, the Windsor majority said the measure created a “second-tier marriage” for gay couples.

“The principal purpose and the necessary effect of this law are to demean those persons who are in a lawful same-sex marriage,” Justice Anthony Kennedy wrote for the majority, in language quoted by the appeals court in the Virginia case. He said the law singled out same-sex couples by “refusing to acknowledge a status that the state finds to be dignified and proper.”

The ruling built on a 2003 Supreme Court opinion, also written by Kennedy, that struck down a Texas sodomy ban.

Justice Antonin Scalia, dissenting from the Windsor ruling, said the majority’s “legalistic argle-bargle” eventually would translate into a ruling legalizing gay marriage. “It is just a matter of listening and waiting for the other shoe,” he wrote.

That process is occurring across the country, faster than perhaps even Scalia expected. In the past year judges have declared gay-marriage bans unconstitutional, or probably so, in New Jersey, New Mexico, Utah, Oklahoma, Virginia, Texas, Michigan, Arkansas, Idaho, Oregon, Pennsylvania, Wisconsin, Indiana, Kentucky, Colorado and Florida.

PUBLIC SUPPORT GROWING

While most of those rulings are on hold, gay couples can now get married in 19 states, up from 12 at the time of the Windsor ruling. The District of Columbia allows same-sex marriage as well.

“What’s distinctive is that the lower courts have actually gone farther than the Supreme Court in massive numbers,” said Eric Posner, a professor at the University of Chicago Law School.

The Supreme Court isn’t above reversing course on an issue and confounding lower courts. In the 1970s, the high court backed away from its embrace of expansive race-discrimination claims, overturning rulings from lower-court judges who thought they were faithfully following the Supreme Court’s lead, said David Strauss, a professor at the University of Chicago Law School.

The difference, Strauss said, was that the Supreme Court’s newfound skepticism of discrimination claims during the 1970s coincided with a similar shift in public opinion.

“The lower courts were increasingly out of step, and the Supreme Court’s reaction was in step,” said Strauss, who teaches constitutional law.

By contrast, public support for gay marriage is growing, reaching a new high of 55 percent in a Gallup poll conducted May 8-11.

Along with public opinion, the deluge of lower-court rulings may have an effect on the Supreme Court, said Walter Dellinger, a Washington lawyer and former acting solicitor general under President Bill Clinton.

“Generally, lower court decisions are not particularly influential with this court, but this case may be an exception,” Dellinger said.

The question for the Supreme Court may be less how it will rule and more whether it will get involved at all during its next nine-month term, which starts in October. Kennedy, Sonia Sotomayor and Ruth Bader Ginsburg have suggested that it’s too soon for the high court to rule on gay marriage.