Eight former secretaries of health, education and welfare declared yesterday that instead of turning welfare over to the states, the federal government should mandate and pay minimum benefits nationwide, eventually as much as $7,000 a year in today's dollars for a family of four without other income.

President Reagan has proposed turning welfare over to the states, but Elliot L. Richardson, HEW secretary in the Nixon administration, denied at a Capitol Hill press conference that the secretaries' recommendation for the Aid to Families with Dependent Children program was an attack on Reagan.

He said the proposal was not intended to pick a quarrel with the president but simply represented the former HEW chiefs' best thinking about the most desirable direction for AFDC. Several of them developed it last year.

Richardson later added, however, "When it comes to the truly needy, we think the nation is losing that sense of responsibility" of which it has long been proud.

In addition to Richardson were Wilbur J. Cohen (Johnson administration), Arthur Flemming (Eisenhower and Kennedy administrations), Patricia Roberts Harris (Carter administration) and David Mathews (Ford administration), all of whom were present at yesterday's press conference, plus Robert H. Finch (Nixon), Anthony Celebrezze (Kennedy-Johnson) and Abraham A. Ribicoff (Kennedy), who sent in endorsements.

Cohen said that under the existing $14 billion-a-year AFDC program, which had 11.1 million beneficiaries as of June, the federal government pays 54 percent, and the states the rest. States are free to set their own levels of benefits. As a result, there are enormous differences: in November, 1981, Mississippi paid $120 a month to a family of four without other income while Vermont paid $601. In addition, there are sharp eligibility differences. The eight proposed:

* Nationwide eligibility standards, with families headed by unemployed fathers eligible everywhere.

* That the federal government set and assume payment of minimum benefits, to be phased in from 1985 to 1991. All families without other income would be guaranteed a combined cash payment and food-stamp benefit of about $7,000 in today's dollars, which is 75 percent of the poverty line. The current poverty line for a family of four is $9,287.

States could supplement this if they wished. Cohen said 24 are now below the 75-percent level.

* To provide work incentives, families with some earnings would be allowed to keep more than allowed under current law without losing benefits.

Once fully phased in, the new proposal would add millions of beneficiaries and billions of dollars to the program.