NEW YORK — Shares of Fannie Mae and Freddie Mac lost more than a quarter of their value on Wednesday as fears mounted that the mortgage financiers will soon need government support and any bailout would hang stockholders out to dry. Since Monday, stock in the two companies — which together hold or guarantee half the U.S. mortgage debt — have plunged more than 45 percent and are now trading at lows not seen in nearly two decades. "There's a big negative feedback loop and there's no way out of it," Friedman, Billings & Ramsey Co. analyst Paul Miller said in an interview. "As the stock falls more and more, it's more likely the government steps in and more likely equity holders get wiped out." Fannie Mae's chief executive sought to reassure investors that no bailout is imminent. "They haven't offered anything and we haven't asked for anything," Fannie Mae CEO Daniel Mudd said in a public radio interview Wednesday morning. "I don't anticipate that they will do that.