House Democrats started a new battle over Social Security benefits yesterday, charging that at least 500,000 low-income people will suffer uncompensated loss of income unless Congress reverses its decision last month to eliminate the minimum benefit as President Reagan requested.

Led by Chairman J. J. Pickle (D-Tex.), Democrats on the Social Security subcommittee challenged assertions by the Office of Management and Budget and deputy Social Security Commissioner Robert J. Myers that scarcely any of the 3 million people currently on the minimum rolls will be hurt badly because they can shift to other benefit programs or have enough other income.

Myers insisted repeatedly that "this really doesn't impact on too many people," but Pickle said that in fact "nobody knows exactly" what the impact will be on nearly half the 3 million, while the administration's own figures project that at least 400,000 to 500,000 low-income people will lose the minimum and not receive welfare or any other compensating payments.

"To take away that little piece of happiness and security that a lot of people this truly a humane thing for a government to do?" asked Rep. Frank J. Guarini (D-N.J.).

The battle over the minimum--the Democrats want to restore it for those already on the rolls--foreshadows an even bigger fight over long-term cuts proposed by the administration to restore system solvency.

On that subject, the Republican-controlled Senate Budget Committee is considering a proposal to limit the annual cost-of-living increases in Social Security and other government retirement programs to 3 percentage points less than each year's increase in the consumer price index, on which the annual adjustment is now based by law. That plan, according to a memorandum by Budget Chairman Pete V. Domenici (R-N.M.), would save the government about $22.4 billion over the next three years.

The Domenici memo also suggested "capping" annual increases in Medicaid payments to the states at 9 percent a year and limiting Medicare increases to 10 percent a year and real defense increases to 7 percent.

Myers, defending elimination of the minimum benefit not only for prospective beneficiaries but for those on the rolls now, said 1.2 million of these would be no worse off because they would be eligible for different types of Social Security benefits that would be no lower if they lost the minimum. Another 500,000 were already also receiving welfare and would have their welfare payments raised automatically when the minimum was cut off.

He said another 500,000 were eligible for welfare already but somehow hadn't applied for it; 160,000 would become newly eligible for welfare; 360,000 were already getting federal or state and local pensions in addition to the minimum Social Security guarantee; 100,000 were married to active workers with substantial earnings, and the remaining 140,000 weren't eligible for welfare but probably had other resources and wouldn't be in hardship.

But Pickle said the administration's own figures showed that it expected only 100,000 of the 500,000 already eligible for welfare but not on the rolls to apply.