Washington ? The Bush administration is scrambling to develop a new Iraq exit strategy with help from the United Nations over the next two to three weeks, but the array of political and security challenges is now so daunting that U.S. officials also quietly acknowledge that the U.S.-led coalition may end up in an even worse position if the latest effort fails.

The United States is banking largely on the negotiating skills of one man to pull it off, U.S. officials concede. After two failed attempts, the administration and its allies are ceding authority to propose solutions to U.N. special envoy Lakhdar Brahimi, U.S. officials said.

“We’re very dependent on him to develop a plan — and then to help legitimize it among Iraqis,” said a State Department official involved in Middle East policy. “This is a time-intensive process, and time’s not something we have a lot of.”

The United States now hopes to return to the United Nations to get a new resolution embracing the transition and possibly giving cover to coalition forces that will remain after the occupation ends. But, with the end of the occupation now less than three months away, several problems in three broad categories will have to be answered first.

Security

Increasing attacks on Iraqi security forces and foreign civilians, like the killings of four private U.S. contractors in Fallujah last week, have made security even more tenuous, U.S. officials say. An area of particular concern is northern Mosul, where security conditions are deteriorating, the officials added.

There are also a growing number of groups targeting military and so-called soft targets. Attacks from Saddam supporters are “trending down,” according to a senior U.S. official familiar with military patterns. But a newer breed of Sunni religious extremists, not members of the former ruling Baath Party or al-Qaida, are “a new element in this picture,” the official added.

Iraq’s well-armed militias are a further challenge in stabilizing Iraq. U.S. governor L. Paul Bremer has a tentative agreement for the disarmament and disbanding of the largest Kurdish and Shiite militias, but the coalition now is bracing for a possible military showdown with the Mahdi’s Army, a militia loyal to Shiite cleric Muqtada Sadr.

All three security problems make it more difficult to get international humanitarian groups, foreign investors, diplomats and even security agencies interested to make commitments to Iraq, U.S. officials say. Ensuring safety is also considered essential in holding elections by year’s end for a provisional national assembly.

Government

Brahimi is now expected to be in Iraq for 10 days to two weeks for initial talks with all major ethnic and religious leaders on the provision government to assume sovereignty when the U.S.-led occupation ends, say officials of coalition countries. He has to find answers to three basic questions: Who will be in the interim government? Who will pick them? And what process will be used? Each question has its own complications.

U.S. officials hope Brahimi will be able to help identify new Iraqi political players, particularly among the Sunni minority that once dominated Iraq but which has been alienated or distanced from government since Saddam’s demise. One of the Bush administration’s biggest disappointments — and concerns — is that few new Iraqi voices have emerged over the past year, leaving members of the Governing Council as the most prominent names despite repeated polls indicating most of them have marginal support.

Constitution

One of the most volatile issues is Iraq’s interim constitution, approved by the Governing Council last month only to be challenged by Shiite and Sunni leaders. The Transitional Administration Law, as it is formally called, was a response to Kurdish demands that their autonomy be enshrined to protect against the Arab majority.

Grand Ayatollah Ali Sistani has been the most serious critic, variously demanding that the TAL be scrapped or not recognized by the United Nations as the basis of Iraq’s law or government. But a legal code is needed for many reasons, U.S. officials say, including restructuring of the $120 billion debt inherited from Saddam’s government with international financial institutions

Brahimi’s most important task may be negotiating with Sistani, although there are as yet no guarantees that the reclusive cleric will again meet the U.N. envoy. Coalition officials hope Brahimi will be able to convince Sistani not to reject the TAL on grounds it could limit economic relief and development.

As part of the new diplomatic effort, Bremer is set to return to Washington this week through the Easter holiday for consultations, U.S. officials say. Robert Blackwill, the National Security Council’s Iraq troubleshooter, will remain in Baghdad to work with Brahimi.