WASHINGTON — President Barack Obama Thursday pledged $100 million and the full resources of… WASHINGTON — President Barack Obama Thursday pledged $100 million and the full resources of the U.S. government to a relief program for earthquake-struck Haiti that he promised would be one of the largest in recent history.

Obama, seeking to make the U.S. effort a bipartisan one, enlisted both former presidents George W. Bush and Bill Clinton to lead the initiative, following an example set by Bush after a tsunami struck Southeast Asia in 2004. It was Obama’s first presidential request of Bush, whom he criticized in the past for Bush’s handling of Hurricane Katrina.

In an emotional appearance at the White House Diplomatic Reception Room, Obama said he had ordered U.S. agencies to make the aid effort one of their top priorities, and promised they would continue to rebuild the hemisphere’s poorest country after the immediate crisis was past.

He said he wanted to speak directly to the people of Haiti.

“You will not be forsaken; you will not be forgotten,” he said. “In this, your hour of greatest need, America stands with you.”

Obama has ordered 2,000 Marines, 3,000 troops from the 82nd Airborne Division and a growing force of ships and aircraft to the island. On Thursday, U.S. military and civilian workers began streaming ashore to step up efforts to rescue the injured, restore damaged infrastructure, and evacuate Americans.

Obama called Bush on Wednesday night and received a promise that he would do all he could for the effort along with Clinton, who has already been serving as a special envoy to Haiti. In 2004, Bush enlisted his father, former President George H.W. Bush, and Clinton to lead the tsunami relief effort.

But private aid officials, while praising the administration’s initial steps, cautioned that the job was immense and complicated, and was likely to run into snags that could subject the administration to criticism.

They predicted the risks of violence and looting would increase as food and water run out, and if the injured fail to find treatment for their injuries. They cautioned that it would be an immense task to coordinate the relief effort so that the government, private aid groups and the military don’t work at cross purposes, as they sometimes did during the tsunami relief effort.

Samuel Worthington, chief executive of Interaction, an umbrella group for U.S. nongovernmental groups that work abroad, said the administration “has been heading in the right direction” in organizing the new effort and trying to coordinate international contributions.

But he acknowledged that for the U.S. government, taking the lead role could mean some criticism because of the difficulty of rescuing the estimated 3 million who are now distressed and injured.

“There are going to be horror stories,” Worthington said. “It’s inevitable.”

Karen Carr of the Community Coalition for Haiti, based in Virginia, praised the administration’s unfolding effort. She said one task will be simply to keep criminal gangs from taking advantage of the situation, and wealthy interests from charging too much for basic commodities.

Former Secretary of State Colin Powell warned on CNN that rescue workers will face a formidable task disposing of the dead. They will need, he said, to treat them with respect, while removing them from the area “and (giving) them a decent burial so that we don’t create an epidemic problem of some kind.”

Pentagon officials said the military effort to help Haiti wouldn’t slow the effort to send 30,000 additional troops to Afghanistan.

Robert Gibbs, the White House press secretary, said the principal focus of the U.S. effort at the moment is to free the large number of Haitians who remained trapped in the rubble.

“We know that there are likely people still trapped in this rubble that have a good chance, if we can get to them,” Gibbs said.

State Department officials said there are now eight search and rescue teams on the ground, from the United States, Iceland, Spain and Chile. Thirty countries have now sent help to Haiti or pledged to do so.

Another focus is restoring communications. In one sign of their poor state, Obama was unable to reach Haitian President Rene Preval in the early afternoon Thursday.

The traffic flow at the Port-au-Prince airport is improving but still limited. Several planes had to hold at the airport or circle in the air Thursday, some for a couple of hours, because the ramp was too crowded.

One or two evacuation flights left Port au Prince Thursday, and there was a chance of a third departure, officials said. Some 300 to 400 Americans were evacuated, they estimated, mostly to the Dominican Republic.

Some lawmakers were raising the possibility that some people could be airlifted from Haiti to the U.S. for medical treatment.

Gibbs said that discussion hadn’t reached the White House, but that there might be some “staging” on the U.S. military base in Guantanamo Bay, Cuba, or on the Comfort, a U.S. Navy ship, essentially a floating hospital.

Given the overwhelming number of injured people, though, Gibbs didn’t rule out the possibility of treatment on American shores.

While many Republican lawmakers joined in voicing support for the administration’s efforts, the effort drew fire from some conservative commentators, bringing return fire from the White House.

Radio host Rush Limbaugh said the disaster gives Obama the chance to show his “compassionate” credentials and to “burnish his credibility with the black community.” He also complained about calls for charitable donations for disaster relief.

“We’ve already donated to Haiti,” he said. “It’s called the U.S. income tax.”

Gibbs responded: “In times of great crisis there are always people that say really stupid things. I don’t know how anybody could sit where he does, having enjoyed the success that he has, and not feel some measure of sorrow for what has happened in Haiti,” he said of Limbaugh.

Aides said Obama didn’t hesitate to call Bush for help, and played down Obama’s past criticism of Bush over Hurricane Katrina.

“He did much better with the tsunami,” said one administration official.