President Bush is expected to face mounting pressure to change his Iraq policy this week, as the U.S. Senate begins debate on military spending priorities.

Democratic leaders in the Senate are planning to introduce a number of amendments to the Defense Department authorization bill, including cutting off funding for combat operations after April of next year and new readiness requirements for U.S. troops. The debate comes as several senior Republican lawmakers have abandoned President Bush on Iraq, saying they can no longer support his strategy.

The New York Times is reporting Monday that debate inside the White House is intensifying over whether Mr. Bush should try to prevent more Republican lawmakers from distancing themselves from him by announcing a gradual withdrawal of U.S. troops from Baghdad and other cities.

The paper cites unnamed administration officials, who say Mr. Bush once thought he had until September to begin discussions on policy changes, but is finding himself under unexpected pressure to act now.

Defense Secretary Robert Gates canceled a four-country tour of Latin America planned for this week to help prepare a report to Congress on the progress the Iraqi government is making. The administration is required to present a report to Congress by July 15, and to present another report on progress in Iraq in September.

In an interview with CNN Monday, Republican Senator Susan Collins said the lack of political progress by the Iraqi government, combined with the tremendous loss of life in Iraq in June, are the main reasons for the "steady erosion of support" of the president's policy among Republican lawmakers.