The Treasury Department is dramatically expanding the scope of its bailout of the financial system with a plan to take ownership stakes in the nation's insurance companies, signaling new concerns about a sector of the economy whose troubles until now have been overshadowed by the banking industry, government and industry sources said. Insurers, including The Hartford, Prudential and MetLife, have pushed the Bush administration to include them in the plan. Many firms have taken losses from mortgage-related securities and other investments and are struggling to replenish their coffers. The new initiative underscores the growing range of problems that Treasury is scrambling to address with the $700 billion allocated by Congress this month. The shape of the plan has changed repeatedly since Treasury Secretary Henry M. Paulson Jr. introduced it last month as an effort to rescue banks by buying their troubled mortgage-related assets. That original mandate has now been pushed aside by a plan to take equity stakes in banks and insurance companies, and other businesses are lobbying to be included. The government has been forced to expand the plan partly because the federal guarantees previously given some institutions, such as banks, have put other companies and financial sectors at a disadvantage, making them less attractive to uneasy investors. The cost of saving the country's largest insurer continues to rise. Senior managers at troubled insurance giant American International Group warned the Federal Reserve yesterday that the company would probably need more taxpayer money than the $123 billion in rescue loans the government has provided.