The Federal National Mortgage Association, the private corporation that holds a portfolio of $34 billion in home mortgages, is back in the business of buying mortgages again.

After unsuccessfully requesting authority for an additional $3 billion of borrowing capacity from the Department of Housing and Urban Development, Fannie Mae recently received authority, in two stages, for an additional $1.4 billion. As a result, FNMA held its regular auction for mortgage commitment purchases to lenders earlier this week.

FNMA also has reopened its "window" for the purchases of mortgages on a 12-month basis. However, it has limited commitments for $5 million in FHA-VA loans and the same amount of conventional loans per seller per week.

As of Jan. 10, the ceiling on Fannie Mae debt and outstanding mortgage commitments was set st $36.5 billion. Now the total is slightly more than $37.9 billion.

At this week's auction, Fannie Mae accepted $637.3 worth of both FHA and VA-conventional mortgages on a four-month commitment basis, during which FNMA is obligated to purchase the mortgages but the lender is not obligated to offer them.

The squeeze on Fannie Mae's authority to extend its limit of mortgage buying has been regarded as part of a political move by HUD to make business life uncomfortable for FNMA and its top officers, including Republican-appointed Chairman Oakley Hunter.

He has been fighting for months to retain the $140,000-a-year post that he holds under a contract that extends into 1980. His post and those of this two top assistants, Lester Condon and Robert Bennett, have reportedly been hotly sought for Democrats by the Carter administration.

On the House floor this week, Rep. B.F. Sisk (D-Calif.) delivered a defense of Fannie Mae against an "unjustifiable course of pressure tactics" by HUD Secretary Patricia Roberts harris. Sisk, who won his seat in Congress by defeating Oakley Hunter some years ago, said that FNMA "can do much to give meaning to private sector involvement in solving our housing problems, and the nation would be well served if Mrs. Harris would get off Fannie Mae's back."

Meanwhile, FNMA has accepted offers to purchase more that $7 million in urban mortgage loans under a new program through which the agency will buy 60 to 90 percent participation interests in pools of mortgages on homes in urban areas where mortgage credit has been tight.

The first two lenders to use the urban program are Independence Savings and Loan Association of Valejo, Calif., and Midwest Federal Savings and Loan Association of Minneapolis. The average loan amounts are under $30,000; most of the mortgaged properties are around 50 years old.