Shares of mortgage giants Fannie Mae and Freddie Mac tumbled again Tuesday as investors began to accept their holdings might be wiped out by a government rescue plan.

Fannie Mae shares fell more than 19 percent to $7.87. Freddie Mac shares declined about 22 percent, to $5.54. Shares of each company have lost about half their value since the beginning of last week.

“Equity holders don’t know where they fit anymore,” Friedman, Billings, Ramsey & Co. analyst Paul Miller said, referring to the government’s plans to help the struggling mortgage financiers.

On Sunday, the Treasury Department and Federal Reserve outlined plans to provide additional lending support to the pair as they struggle with mounting losses tied to rising defaults and waning investor confidence.

The Treasury will expand its line of credit for the pair and is asking Congress to allow it to purchase an equity stake, if necessary, in the companies. The Fed has opened a lending option to Fannie and Freddie that was previously only open to retail and investment banks.

Fannie and Freddie hold or guarantee more than $5 trillion in mortgage debt, which is nearly half of the nation’s mortgage debt. The pair are considered vital to helping the struggling real estate market rebound.

On Tuesday, the Securities and Exchange Commission added to the regulatory effort to help Fannie and Freddie. The SEC announced an emergency rule aimed at stopping “naked” short selling of Fannie and Freddie shares, along with shares of investment banks.

Short sellers borrow shares of stock and sell them. If the price drops, they purchase actual shares to cover the borrowed ones. Short sellers are betting prices will drop on a stock so they can turn a profit.

“Naked” short selling is when sellers do not even borrow the shares before selling them, and then look to cover positions immediately after the sale. The new SEC rule would require short sellers to actually borrow shares before selling them.

Short selling has been blamed for a sharp decline in the price of Lehman Brothers Holdings Inc. shares and was considered part of the reason for the collapse of Bear Stearns.

While the government support helps enable the long-term viability of the companies, Goldman Sachs analyst Daniel Zimmerman wrote in a research note that the pair still need more capital.

“Depending on terms, such an infusion could prove dilutive to equity shareholders,” Zimmerman said.

Also Tuesday, Moody’s cut its preferred stock rating on both Fannie and Freddie to “A1″ from “Aa3.” Fannie and Freddie’s bank financial strength ratings were cut to “B-” from “B.” Both ratings remain on review for a potential downgrade.

The ratings were cut because of diminished financial flexibility for the pair as credit markets continue to weaken. Moody’s did affirm Fannie and Freddie’s senior long-term debt at “Aaa” because of the government support.