WASHINGTON — A fixture since President Harry Truman signed a bill proclaiming the National Day of Prayer 58 years ago, this year could mark the last time the event is observed if the White House fails in an appeal against a court ruling that it violates the ban on government-backed religion.

Wisconsin-based U.S. District Court Judge Barbara Crabb ruled last month in favor of the Freedom From Religion Foundation in a suit brought against President Obama. She ruled that the federal law that designates the first Thursday in May as a National Day of Prayer and requires an annual presidential proclamation violates the establishment clause of the Constitution's First Amendment.

Despite that ruling, several observances took place around the capital Thursday, including at the Pentagon and on the steps of the U.S. Capitol.

In her decision, Crabb said until the defendants in the case exhaust their right to appeal the decision, observance ceremonies could still go ahead.

Charles Haynes, a senior First Amendment scholar specializing in religious liberty at the Washington-based Freedom Forum, said he expects the president to succeed with his appeal. But he said Crabb was merely being consistent with past rulings and saying "what everybody knows about the inherent (constitutional) contradiction of the National Day of Prayer."

"The courts are not immune to public dissatisfaction," he said, adding the Court of Appeal could possibly cite a 1983 Supreme Court decision that upheld the right to legislative prayer, on grounds "the offering of prayer is a tolerable acknowledgment of beliefs widely held among the people of this country."

"It is possible the courts would argue that the Day of Prayer goes all the way back to our Founding Fathers and that they are not going to disturb it," he said. "Prayer, in this case, is merely 'ceremonial deism.' "

Meanwhile, evangelist Franklin Graham prayed on a sidewalk outside the Pentagon Thursday after the Army decided two weeks ago to rescind an invitation to its National Day of Prayer service following an objection by the Military Religious Freedom Foundation that Graham has insulted people of other religions, particularly Islam.

Graham is the honorary chairman of the private National Day of Prayer Task Force, which leads national events where all the prayers are strictly Christian.

The Army said Graham's past comments were "inappropriate as a speaker for an open religious service."

Haynes said the National Day of Prayer was an attempt to unify people of different faiths, but fights have become more common in part due to the prominence that the George W. Bush administration accorded to private evangelical Christian groups such as the National Day of Prayer Task Force, fostering perceptions that the government was endorsing a sectarian religious approach.