Three federal judges have now ruled that state bans on same-sex marriage violate the Constitution’s “equal protection” clause. President Barack Obama seems to disagree.

He has repeatedly stated that, while he personally supports same-sex marriage, the issue should be left to the states. In other words, the nation’s first black president holds a states-rights position on what has shaped up to be the civil-rights issue of this generation.

Most states ban same-sex marriage. And if they are left to work out the issue for themselves in the years and decades to come, some will likely reverse their bans, but many won't. The question will remain: Are same-sex marriages protected under the 14th Amendment?

It’s hard to imagine that Obama, a former constitutional law professor who has cited the 14th Amendment when discussing gay rights, believes that the equal-protection clause does not apply to gay and lesbian couples seeking a government-issued marriage license. More likely, he has taken a go-slow approach to avoid alienating constituencies -- and inflaming the opposition.

His caution has been understandable, perhaps even helpful in the short run. But it will become increasingly difficult for him to avoid taking a clear stand. Marriage equality is likely to come before the U.S. Supreme Court before the end of his term, possibly within the next year. At that time, Obama will have to decide whether to support the plaintiffs.

He should, and he should not wait for the court to force his hand.

The presidential bully pulpit has been a powerful force in accelerating the evolution of freedom and equal rights in America, and the energy with which presidents have used their platform -- or not -- shapes their legacy.

In 1957, President Dwight Eisenhower declined to speak out in favor of civil rights, saying, "I don't know what another speech would do about it right now." In response, Martin Luther King Jr. and the Southern Negro Leaders Conference sent a telegram urging him to “use the weight of your great office to point out to the people of the South the moral nature of the problem.”

While a strong case can be made that history has so far underappreciated Eisenhower’s record on civil rights, his unwillingness to make energetic use of his office is widely accepted as a failure.

President John F. Kennedy is still faulted for waiting two and a half years to fully embrace the cause of civil rights. When he finally did, in an Oval Office speech in June 1963, he cast the issue in terms of morality and American values. And while this may have angered opponents, it inspired more hope than hate. It became a turning point in the civil-rights movement and saved his presidency from history’s harsh judgment.

This Congress is hardly likely to pass a law prohibiting discrimination in the granting of marriage licenses. But that should not stop this president from addressing a controversial question. If Obama believes that the 14th Amendment protects same-sex marriage, he should explain why he has changed his own mind. It is an argument that Americans -- who polls show favor legalizing same-sex marriage -- are increasingly ready to accept. If he makes the case in a tone of respect toward opponents, and if he tempers proponents' optimism by acknowledging the lengthy nature of civil-rights movements, he can help soften the fallout from the eventual Supreme Court decision, whatever it may be.

For a president so stymied by an obstructive Congress, an Oval Office address on same-sex marriage could well be a high point of his tenure. It would also be a fitting way for a barrier-breaking president to secure his place among leaders who have pushed to extend the full rights of the Constitution to all Americans.

To contact the editor responsible for this article: David Shipley at davidshipley@bloomberg.net.