The government has just become one of the biggest players in the US mortgage market. The Bush administration announced Sunday it was seizing troubled mortgage giants Fannie Mae and Freddie Mac in a bid to help reverse a prolonged housing and credit crisis. But private analysts worried that it may not be enough to stabilize the slumping housing market given the glut of vacant homes for sale, rising foreclosures, rising unemployment and weak consumer confidence. Mark Zandi, chief economist at Moody's Economy.com predicted that 30-year mortgage rates, currently averaging 6.35 percent nationwide, could dip to close to 5.5 percent. That's because investors will be more willing to buy the debt issued by Fannie and Freddie - and at lower rates - since the federal government is now explicitly standing behind that debt. “Effectively, the federal government has now become the nation's mortgage lender,” he said. “This takes a major financial threat off the table.” Officials announced that both Fannie Mae and Freddie Mac were being placed in a government conservatorship, a move that could end up costing taxpayers billions of dollars. Treasury Secretary Henry Paulson refused to estimate how much the takeover of the two companies will cost the government, but he insisted that taxpayers will get paid back first. “We structured this facility to protect the taxpayer,” Paulson said Monday in an interview on the CBS Early Show. “The government will be repaid ... before the shareholders of these companies get a penny.” In a separate appearance on CNBC, Paulson said “we obviously don't know” when asked how much the takeover could end up costing taxpayers. He said that will depend on how quickly the housing market turns around. Wall Street posted a huge rally Monday as investors reacted with enthusiasm to the government's actions. The Dow Jones industrial average was up nearly 250 points in late morning trading. Foreign investors own about $1.5 trillion of the debt issued by Fannie, Freddie and smaller agencies such as Ginnie Mae last year and are likely to pile up billions more in losses until the housing market begins to recover. The Treasury Department said it was prepared to put up as much as $100 billion over time in each of the companies if needed to keep them from going broke, in exchange for senior preferred stock. Treasury will immediately be issued $1 billion of such stock from each company, which will pay 10 percent interest. Further purchases of preferred stock will be triggered if quarterly audits find that the companies' capital cushion is below prudent standards. The government, which will receive warrants representing ownership stakes of 79.9 percent in each company, is hoping that its moves will reassure nervous investors that they can continue to buy the debt of the two companies. In a statement, President George W. Bush said, “Americans should be confident that the actions taken today will strengthen our ability to weather the housing correction and are critical to returning the economy to stronger sustained growth.” The conservatorship will be run by the Federal Housing Finance Agency, the new agency created by Congress this summer to regulate Fannie and Freddie, a move taken at the same time that Congress greatly expanded the power of the Treasury Department to make loans to the two companies and purchase their stock. The executives and board of directors of both institutions are being replaced. Herb Allison, the former head of the TIAA-CREF retirement investment fund, was selected to head Fannie Mae, and David Moffett, a former vice chairman of US Bancorp, was picked to head Freddie Mac. Paulson was careful not to blame Daniel Mudd, the outgoing CEO of Fannie Mae, or Freddie Mac's departing CEO, Richard Syron, for the companies' current problems. While both men are being removed as the top executives, they have been asked to remain for an unspecified period to help with the transition. Fannie and Freddie both purchase home loans from banks and then repackage those loans as mortgage-backed securities that they either hold on their own books or sell to investors around the globe.