The main thrust of President Carter's new urban strategy should be to target federal assistance in the handful of cities for which "it is a matter of life and death." Secretary of Housing and Urban Development Patricia R. Harris said yesterday.

Saying she is ready to battle budget officials for more aid, Harris said in an interview she also is convinced "we can't just throw the money up for grabs" between those cities that need it for survival and those for which "it would be a convenience."

Harris hinted strongly that she would recommend shifting the $2.25 billion in the "countercyclical aid" program, scheduled to expire next year, into a more tightly targeted attack on hardcore unemployment and deteriorating neighborhoods in the most seriously afflicted cities.

She said that various measures used by her department put from eight to 22 cities on the list of those with critical fiscal problems, but HUD officials said later they were not certain which cities fit that description.

One list in a memorandum referred to by the Secretary includes Baltimore, Boston, Denver, New Orleans, Philadelphia, St. Louis, San Francisco and Washington - but not New York City, Newark, Cleveland or Detroit, which are usually listed among the most distressed major cities.

Whatever the specifies, Harris indicated she would urge the President in the urban policy memo she is scheduled to send him within the next week to focus federal assistance more tightly than in the past on the worst off cities.

"The targeting issue is the one wer ought to be debating," she said.

The secretary said that both the community development block-grant amendments approved this year and the new urban development action grants are focused on the needs of distressed cities. "The question is whether we can move faster" in relieving their fiscal burden, she said, by shifting existing funds and "targeting" the new money she hopes to obtain.

Referring to an article in Saturday's Washington Post real estate section that her budget request for fiscal 1979 had been rejected as too expensive, Harris acknowledged that "obviously there are some differences between me and some of the first-level budget-makers."

But she said he request had not yet been reviewed either by James T. McIntyre Jr., acting director of the Office of Management and Budget, or the President, and said, "The question is what policies will prevail."

She flat ly denied that she had threatened to resign if her requests were seriously trimmed, saying that would be a "stupid" tactic when she was still engaged in negotiations with the White House.

Harris also took a swipe at the controversial "new towns" program, hinting strongly that she would recommend killing or substantially revising it.

That program, which began in the Nixon administration in 1970, is aimed at building new communities without the problems that plague some older cities. Seven of the 13 projects have gone broke and others have been beset with troubles.

Harris pointedly noted that "this administration and its Democratic predecessors have no responsibility" for the new towns program and said, "I am very concerned about a program that would accelerate the movement from the center city to new suburbs."

She said she would announce her plans for the new towns program soon.