WASHINGTON (AP) President Bush, eager to deploy a missile shield long sought by Republicans, soon will give Russia notice that the United States is withdrawing from a landmark 1972 arms-control treaty, U.S. government officials said yesterday. The pact bans missile defense systems.

Bush will invoke a clause in the Anti-Ballistic Missile treaty that requires the United States and Russia to give six months" notice before abandoning the pact, the sources said. Initial White House plans were to announce the decision tomorrow, but officials cautioned the date could change. One source said formal notice would be issued in January. The four government officials spoke on condition of anonymity.

"The time is coming when we will need to move beyond the ABM treaty," said Sean McCormack, a White House spokesman. Last week, a group of Russian military officials on a visit to Washington told private American arms-control experts they expected the Bush administration to give notice of withdrawal over the year-end holidays.

With the decision, Bush takes a huge step toward fulfilling a campaign pledge to develop and deploy an anti-missile system that he says will protect the United States and its allies, including Russia, from missiles fired by rogue nations.

Bush has said the Sept. 11 terrorist attacks heightened the need for such a system.

Russia and many U.S. allies have warned Bush that withdrawing from the pact might trigger a nuclear arms race. Critics of the plan also question whether an effective system can be developed without enormous expense.

Conservative Republicans have urged Bush to scuttle the ABM, rejecting proposals to amend the pact or find loopholes allowing for tests.

The chairman of the House Armed Services Committee, Bob Stump (R-Ariz.), said he has received no advance tip from the administration, but he backs the plan.

"There"s all these questions about Russia upholding their end of the treaty anyway, and I just don"t think we should penalize ourselves," Stump said. "We shouldn"t delay our ballistic missile defense. If it takes withdrawing from the ABM treaty, that"s fine."

Senate Majority Leader Tom Daschle (D-S.D.) told CNN he was opposed to pulling out of the pact. "It is not a good idea. It would be a real setback for defense and foreign policy to violate the ABM treaty." He added: "It"s a slap in the face for many people who have committed years if not decades" to arms control.