Organizers say this year's National Day of Prayer may have more participation than any other in recent history now that the observance is under fire.

"I really believe this is going to be one of the strongest—if not the strongest—showing and outpouring of prayer that you've seen for an observance of the National Day of Prayer (NDOP) in our history," said Michael Calhoun, director of strategic communications for the NDOP Task Force.

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Calhoun did not give specific numbers, but he estimated there would be tens of thousands of gatherings nationwide Thursday, with millions of Americans participating, including several members of Congress. He said with two wars, high unemployment, recent natural disasters and the ongoing threat of terrorism, Americans believe the nation needs prayer. But a federal judge's recent ruling that the prayer day is unconstitutional is also a motivating factor.

"Though [the ruling] doesn't cancel this year's event, it does threaten to remove it in the future, which I think a lot of Americans are concerned about, as they should be, because this is really an attack on the religious heritage of Americans," Calhoun said.

The NDOP Task Force has partnered with the Alliance Defense Fund (ADF) for a Save the National Day of Prayer campaign, which has more than 250,000 supporters on its Facebook page. Although the Obama administration is appealing the judge's decision and the president already has issued a proclamation for Thursday's observance, the groups are encouraging the public to sign an online petition calling on President Obama to strongly support the prayer day.

"[The petition says] we're watching, and we want the Obama administration to vigorously defend the National Day of Prayer, using every avenue at its disposal to make sure that this is a day that we can pass on to our children and our grandchildren," Calhoun said.

The American Center for Law and Justice (ACLJ) has launched a similar online petition aimed at preserving the NDOP. The Christian legal group plans to file an amicus brief in the appeals case on behalf of 50 members of Congress and 30,000 individuals.

"That is just to show there is a real, 50-state interest in making sure these time-honored traditions stay part of our heritage," said ACLJ Chief Counsel Jay Sekulow. His group also represented 30 members of Congress in the original lawsuit, filed in 2008 by the Wisconsin-based Freedom From Religion Foundation.

Sekulow and other legal observers expect the prayer day to prevail, but they say the case may be appealed to the Supreme Court before that happens.

In her April 15 ruling, Wisconsin U.S. District Court District Judge Barbara B. Crabb wrote that the prayer day violates the Establishment Clause because it does more than acknowledge religion. She said its purpose "is to encourage all citizens to engage in prayer, an inherently religious exercise that serves no secular function in this context." (Read the decision.)

Sekulow worries that the Seventh Circuit Court of Appeals may be sympathetic to Crabb's view based on its rulings in similar cases. But prayer day supporters are optimistic the Supreme Court will uphold the observance because of a precedent set in its 1983 Marsh v. Chambers ruling, which declared legislative prayers constitutional.

Crabb's decision confused some prayer day supporters, who thought this year's events were cancelled as a result of the ruling. Last month, ADF sent a letter to city mayors informing them that Crabb's decision does not take effect until after the appeals process is complete and should not interfere with their 2010 plans.

But ADF attorney Joel Oster said it is still possible that some city and state officials may be challenged legally for their participation in NDOP events. "The tradition of our nation has been to allow different official prayer proclamations ... and there are just a lot of groups out there that do not like that history, and so they're actively seeking to change our history," said Oster, who was part of the ADF team that represented NDOP Task Force Chairman Shirley Dobson in the 2008 lawsuit, which named her as a defendant.

Oster said the Freedom From Religion Foundation is against religion altogether, not just government endorsements of prayer. "They're not like one of these groups that is out there for some purist separation of church and state kind of concept," Oster said. "They actually want religion to stop."

Even if the prayer day is upheld, some Christians say Crabb's ruling highlights a growing hostility toward Christianity that could lead to religious persecution. Although the Pentagon is marking the prayer day Thursday with a multi-faith service, it recently revoked its invitation to the NDOP Task Force and their honorary chairman, Franklin Graham, because he has called Islam "a very evil and wicked religion."

Graham, son of evangelist Billy Graham and founder of the humanitarian ministry Samaritan's Purse, has stood by his comments. He says his view is based on decades of ministry in the Middle East, where he has seen Islam fuel religious violence and the mistreatment of women. He says the Pentagon's decision does not bode well for Christians.

"I think no question ... religious freedom is under attack," Graham told The 700 Club in a recent interview. "There has been an erosion now for many years, but we have seen it really accelerate in the last 10 years.

"This political correctness that has crept in, that if we stand for what we believe in, [all of a] sudden we are not tolerant," he added. "They almost make it look like we are participating in hate speech when we say that Jesus Christ is the way, the truth, and the life, and there's no way to God except through Christ and Christ alone. They are interpreting that now as being hostile and hate speech."

Graham will be the keynote speaker a NDOP observance in Washington, D.C., Thursday. The prayer service will be broadcast live from 9 a.m. until noon at the NDOP Web site and on God TV.