WASHINGTON — The two-decade drive to legalize same-sex marriage passed a major milestone Monday and neared final victory as the Supreme Court cleared the way for gays and lesbians to marry in 11 additional states.

Social conservatives had seen the high court as their last hope to halt gay marriage, but in a surprise move justices flatly turned down appeals from five states, some with right-leaning state attorneys who were seeking to reinstate bans against same-sex marriage.

Instead, the high court let stand appellate rulings that had struck down the bans, therefore putting off once again the thorny question of whether gays and lesbians nationwide have a constitutional right to marry.

But even though the action, issued with no comment, does not carry the weight of a formal ruling, it is likely to send a strong message to lower court judges that same-sex marriage could soon be legal across the country.

Monday’s action means gay marriage will be legal in 30 states, including for the first time in the South. The court order makes gay marriage immediately legal in Indiana, Oklahoma, Utah, Virginia and Wisconsin. Not far behind will be six other states in the same federal circuit districts where gay marriage bans are now in effect illegal: Colorado, Kansas, North Carolina, South Carolina, West Virginia and Wyoming.

Added to the 19 states that had legalized same-sex-marriage, the court’s move brings gay marriage to most of the nation for the first time.

The ACLU’s James Esseks called it “a watershed movement for the entire country.”

“We are one big step closer to the day when all same-sex couples will have the freedom to marry,” he said.

In the end, conservatives on the court may have decided that the battle over gay marriage is not worth fighting at this time. Public opinion polls show growing support for same-sex marriage. Since the high court’s last ruling on the issue in 2013, same-sex-marriage advocates have won an unprecedented string of legal victories.

The odd outcome may also reflect a close split and uncertainty within the high court.

Chief Justice John G. Roberts Jr. has often shown a willingness to put off decisions on divisive social issues, particularly when the court is closely split. Last year, for example, the justices refused to hear several cases testing new state regulations on abortion.

The four most conservative justices are believed to oppose the lower court rulings in favor of gay marriage. But at least some of them, including perhaps Roberts, appear to have been reluctant to accept a case at this time, perhaps out of concern that swing-vote Justice Anthony M. Kennedy might join with the four liberals to rule for a nationwide right to gay marriage.

At the same time, the four liberal justices see no compelling reason to hear the gay marriage appeals at a time when courts are overwhelming ruling in favor of gay marriage.

But that may change soon. Judges from the 6th Circuit Court of Appeals based in Cincinnati heard an appeal recently and hinted they may well uphold traditional marriage laws in Ohio, Michigan, Kentucky and Tennessee.

The U.S. appeals courts in New Orleans, Atlanta and St. Louis have yet to rule on the constitutionality of gay marriage. If one of them upholds the state bans, gay rights lawyers will appeal, and the Supreme Court is then nearly certain to hand down a constitutional ruling. The justices will not allow the Constitution’s guarantee of “equal protection of the laws” to mean one thing in one region and something different in another.

Monday’s court action was greeted as an important win by gay rights advocates, but not the nationwide ruling they had hoped for.

“This is historic. The Supreme Court has chosen not stand in the way of love,” said Jon Davidson, legal director for the Lambda Legal advocacy group. “Now, we need a national solution so that, no matter where you live, same-sex couples can have access to the dignity and respect that only marriage can provide.”