President Carter will send his plan to revise the nation's welfare system to Congress on Saturday, despite urgings from two powerful lawmakers to take a second look at the package, White House sources said yesterday.

Sen. Russell B. Long (D-La.), chairman of the Senate Finance Committee, yesterday joined Rep. Ac Ullman (D-Ore.), chairman of the House Ways and Means Committee, in asking Carter to delay announcing his plan. Long, advocating tougher work requirements than the Carter plan envisions, "I don't like the concept that some people like mothers are not expected to worl."

Carter's proposal would exempt some mothers with dependent children. Prior to a meeting with Long, the administration had settled on a recommendation that a woman be considered able to work if she has no children under 14. Long favors the age of six.

Carter met yesterday afternoon with Health, Education and Welfare Secretary Joseph A. Califano, Jr., and aides on the welfare package. The President's announcement is expected to come Saturday after Congress adjourns for summer recess and will be made in Plains, Ga.

Carter told Long yesterday he was "ready to start the long tougher regulations . . .and public hearings."

Carter's proposal places strong emphasis on jobs - 1.1 million to 1.4 million public jobs would be created - and would providie work incentives by giving welfare payments to the working poor based on their income and the size of their family. In addition, the amount of income that would be "disgraded" before being taxed would be higher if the person were employed in a private job rather than a government-created public job.

The plan would "cash out" the food stamp program and provide cash assistance to the working poor as well as those not expected to work.

The proposal estimates that 2 million more people would be served by the new Carter plan than the current 30 million. The current annual welfare cost in estimated at $25.8 billion. The new proposal would cost $3.1 billion - but the additional $3.1 billion would be offset by several measures if Carter follows HEW recommendations.

Among these offsets would be increases Social Security payments from those who would be working and decreased umemployment insurance outlays. Other HEW suggestions for saving money included gains through moves to reduce welfare fraud and abuse plus revenues from wellhead tax revenues in Carter's new energy program.

One large expense would be an increased cost in the earned income tax credit. The HEW recommendation specified this expenses would, in part, be defrayed through a change in the current tax system. The earned income tax credit reduces taxes for low-income people. It now phases out completely at $8,000. The administration would phase it out at a higher income about $16,000 for a family of four.

The administration's basic welfare benefit for a family of four with no one expected to work would be $4,200. Those not expected to work include the aged, blind and disabled, and mothers with dependent children.

Two plans in the proposal that would, in effect, tighten eligibility are certain to be controversial.

The first would redefine the "filing unit" - what constitutes a "family" eligible for welfare. The proposal is expected to count all blood relatives living in a household - and their income - when calculating welfare benefits.

It is estimated this definition would save $1 billion over the present aid to families with dependent children program - which counts AFDC mothers and their children separately from other household members. Critics say the new definition would drastically reduce assistance to many of the poor.

A teenage mother with a baby and no income would become ineligible, for example, if she lived with an aunt whose earnings exceeded the welfare cutoff - even though the aunt is not obligated to support the teenager. Carter's plan include some review procedure so that in such households the persons could file separately if the aunt could prove she did not supports her niece.

The other plan to tighten eligibility would extend the "accounting period" used to calculate eligibility for benefits. The current system operates on a "prospective" accounting period - with those seeking assistance estimating their anticipated income. If they are laid off, for example, and anticipate little or no income, they can get aid.

The administration proposal is expected to change the system to a "retrospective" program that would compute what an individual actually earned in the last six months. Critics charge that many would have to wait months to receive any assistance if there was anything left from previous income, but proponents say the new plan reduces the number of eligible recipients and therefore saves money.