Senior Bush administration officials are completing a plan that could help up to three million homeowners struggling to pay their mortgages to stay in their homes, three people briefed on the proposal said Wednesday.

The initiative could be the most sweeping government effort directed at mortgage borrowers since the financial crisis began last year. Under the plan, the government would agree to shoulder half of the losses on home loans if mortgage companies agreed to lower borrowers' monthly payments for at least five years, according to the people briefed on the plan who asked not to be named because details were still being negotiated.

Officials from the Treasury Department and the Federal Deposit Insurance Corporation are working on the proposal and an announcement may come soon. Sheila C. Bair, the chairwoman of the F.D.I.C., has been the leading proponent of the plan and first discussed the idea publicly a week ago.

The plan could cost $40 billion to $50 billion and would be part of the $700 billion financial bailout package that Congress approved earlier this month. The money would go toward covering future losses on loans that are modified according to standards established by the government.