The Reagan administration is reportedly planning to cut another $1.5 billion from Labor Department programs, including 15 percent from Job Corps funding, as it scrambles to close to fiscal 1982 gap that has suddenly grown $9 billion to $12 billion and possibly even more.

The Reagan budget cutters have also reportedly taken fresh aim at the Agriculture Department's supplemental nutritional program for low-income women, infants and children, the so-called WIC program, and that department's "target-price" program to supplement incomes of farmers when prices fall below specified support levels. Budget officials are reportedly looking for $1.8 billion in cuts from the Agriculture Department beyond those proposed by President Reagan last week.

In addition, administration budget officials have told the Education Department to reduce spending for elementary and secondary school programs by 25 percent, rather than the 20 percent originally proposed, officials said. Under the Reagan plan, these education programs are to be consolidated into separate block grants to states and localities, as well as cut.

To hit their spending and deficit targets, administration budget officials -- who last week identified some 83 programs for cuts totaling $35 billion -- are now having to dig even further than planned into the budget for the 1982 fiscal year that begins Oct. 1. Last week, the administration said that, on top of the first $35 billion, it would have to find $6.5 billion more in reductions by March 10 to meet its goals. Earlier this week it said it had miscalculated and needed $9 billion to $12 billion. Now congressional sources say even that figure is too low, and certain to grow larger.

It also became clear yesterday that the administration is facing a problem on the revenue side of the budget. Source said the administration had been counting on an increase in the federal gasoline tax to raise $2 billion in new tax revenues in fiscal 1982. That increase was not specifically mentioned in the budget document put out last week; Reagan budget director David A. Stockman and Transportation Secretary Drew Lewis first mentioned it in a session with governors this week.

The next day, however, Reagan emphatically ruled it out, and, in fact said he had never heard of it.

Administration officials, including Treasury Secretary Donald T. Regan, then said it had only been a Stockman-Lewis idea, and never a presidential recommendation.

Regan yesterday sought to blame the Carter administration for the budget gap, saying it had underestimated spending levels for 1982 and the Reagan administration did not discover this until after the president's Feb. 18 announcement of his program Carter budget officials "had, if you'll pardon my expression, too rosy a scenario" for government spending, Regan said.

But presidential adviser Martin Anderson and other officials say that criticism is wrong. They said most of the new budget gap is due to the effect of unanticipated inflation on such basic aid programs as Medicare since the Carter estimates were made nearly three month ago.

As the new round of budget cutting proceeded in the administration, the political backlash against spending cuts continued to build.

A group of 30 labor unions, religious, public interest and consumer organizations representing 20 million members said it is urging the House and Senate to resist proposed spending reductions income-support programs, saying that would "severely harm the poor and working poor of this nation, yet will have an insignificant impact on inflation." The groups will hold a new conference today.

Support for the president's program remained solid among House and Senate Republicans, however. Senate Republican leader Howard H. Baker Jr. (Tenn.) said the increase in the spending reduction goal would not delay action on the Reagan plan.

Meanwhile, the Senate Budget Committee, concerned that President Reagan's plan may leave too large a budget deficit, is studying cuts even beyond the administration's targets. Changes in 65 programs are being reviewed, including cost-of-living adjustment formulas for Social Security and other federal retirement programs.

The administration's reported plan for a 30 percent cut in the nutrition program for women, infants and children would mean a loss of supplemental food for about 700,000 recipients -- primarily low-income pregnant and breast-feeding women and their young children. It was not clear how many would be provided for from other federal food programs.