WASHINGTON — The government’s takeover of Fannie Mae and Freddie Mac is meeting with a skeptical reception from some onlookers who are warning that the move could leave taxpayers on the hook for billions of dollars in losses.

Treasury Secretary Paulson announced a four-part plan yesterday in which the government would inject capital, guarantee home loans, and buy up to $5 billion in mortgages in an attempt to stabilize the companies, which have lost $14 billion in the last year. Mr. Paulson said he had fired the chief executives of the companies and replaced them with two veteran financial leaders, Herbert Allison at Fannie Mae and David Moffett at Freddie Mac.

The Treasury Department has tasked the newly created Federal Housing Finance Agency to control and oversee the firms, establishing a conservatorship with the goal of keeping them solvent. Mr. Paulson would not disclose how much the takeover could cost the government, saying only that the “ultimate cost to the taxpayer will depend on the business results” going forward for the government-sponsored enterprises.

As he announced the move, the secretary blamed the turmoil surrounding Freddie and Fannie on their “flawed business model” and said Congress and the next president would have to resolve the “inherent conflict” posed by their status as a hybrid of public and private entities.

While the move drew tempered praise from top lawmakers and the two major presidential candidates, it faced criticism from economists who argued against propping up companies that Mr. Paulson had already condemned as defective. “It’s a bad plan, almost an inexplicable plan,” an analyst of financial markets at the American Enterprise Institute, Peter Wallison, said. “If they are in fact flawed business models, they ought to be put out of business.”

Criticizing the conservatorship as “a gift” to shareholders, Mr. Wallison had urged the Treasury Department to appoint what’s known as a “receiver” for each company. Under a receivership, Mr. Wallison said, the shareholders would be wiped out and managers would be able to concentrate solely on minimizing losses to the taxpayers.

The current plan, he said, would lead to larger losses for the government. “Nobody really knows what the losses will be, but they will be substantial,” Mr. Wallison said.

Other analysts who support privatizing Fannie and Freddie said the government had little choice. “It adds a certain clarity to the situation. Before they were sort of flapping in the wind,” the founder and chief executive of Wall Street Strategies, Charles Payne, said.

“I would like to see them privatized at some point, but they were just too deep in the abyss to compete right now,” he said, adding that while a bailout rewards bad behavior by investors, “the stakes are just too high” to let the firms fail. “The doomsday clock was ready to strike midnight for both these companies.”

The managements of Fannie and Freddie have also been criticized for cronyism and their reliance on lobbying lawmakers in Washington.

A former chief economist at the U.S. International Trade Commission, Peter Morici, praised the plan but faulted Mr. Paulson for bringing in two longtime Wall Street executives in Messrs. Allison and Moffett rather than officials from outside the New York-Washington axis. Mr. Allison has been a vice chairman of Merrill Lynch and Mr. Moffett the vice chairman of U.S. Bancorp.

“The new faces look like the old faces,” Mr. Morici said. “This is more of the same from this administration: a good plan poorly executed.”

Despite the criticisms, the analysts expected the actions to boost the stock markets at this morning’s open. “The market usually goes up when the government acts,” Mr. Wallison said.

Fannie and Freddie have assets of $5 trillion, and they now guarantee about 70% of new home loans. Policymakers have been nearly unanimous in warning that a failure of the companies would roil the entire American economy. “Fannie Mae and Freddie Mac are so large and so interwoven in our financial system that a failure of either of them would cause great turmoil in our financial markets here at home and around the globe,” Mr. Paulson said. “This turmoil would directly and negatively impact household wealth: from family budgets, to home values, to savings for college and retirement. A failure would affect the ability of Americans to get home loans, auto loans, and other consumer credit and business finance. And a failure would be harmful to economic growth and job creation.”

Under the plan, the Treasury will provide up to $100 billion to each company to cover future capital shortfalls in exchange for $1 billion in senior preferred stock. The firms will be able to modestly expand their mortgage and mortgage-backed securities portfolio until the end of 2009 before they will be required to reduce their size annually by 10%.

The two major presidential campaigns offered tempered praise for the takeover. Senator Obama said in a statement that he believed “some form of intervention is necessary” and would be reviewing the plan. Senator McCain, in an appearance on CBS’s “Face the Nation,” also offered support, though he criticized the “sprawling, massive bureaucracy” that he said he contributed to the crisis. “I think it has to be done,” he said. “I think that we’ve got to keep people in their homes. There’s got to be restructuring, there’s got to be reorganization and there’s got to be some confidence that we’ve stopped this downward spiral.”

The move drew a blistering response, however, from a third-party candidate, Robert Barr, who warned that the bailout could cost taxpayers $100 billion or more.

“Fannie Mae and Freddie Mac need to be privatized and sold off,” Mr. Barr, the Libertarian Party nominee, said in a statement. “And Congress must stop treating the federal government like a national soup kitchen for businesses in trouble.”