House Judiciary Committee Chairman Peter W. Rodino Jr. (D-N.J.) charged yesterday that the Senate plan to force a balanced federal budget by 1991 is unconstitutional because it improperly delegates legislative authority to the president.

The plan, now before a House-Senate conference committee, sets fixed targets for reducing the federal deficit and requires the president to make proportional cuts in spending if Congress fails to meet the targets.

In empowering the president to cut back programs already approved by Congress, "the proposal attempts to authorize the president to undo a law by something less than a law, and is thus unconstitutional," Rodino said in a letter to the conferees.

The constitutional argument is one of the principal objections raised by House Democrats to the Senate-approved plan, along with fears over its economic effect in a recession and charges that its impact has been softened to protect Senate Republicans in next year's elections.

But the House, while it has not acted on the Senate proposal, has endorsed the plan's goals and Democratic leaders have pledged to try to work out a compromise in the conference.

Asked about Rodino's objections, Senate Finance Committee Chairman Bob Packwood (R-Ore.), chairman of the conference committee, said that the "constitutional arguments are a smoke screen to avoid the issue of deficit reductions ."

Packwood and Senate Budget Committee Chairman Pete V. Domenici (R-N.M.), who, like Rodino, are lawyers, contend that the plan is sufficiently specific in its grant of powers to the president to meet constitutional tests.

"Those who want to nitpick it to death . . . we'll never satisfy them," said Packwood.

In his letter, Rodino raised questions over policy, but he said that his objections primarily were constitutional, involving the separation of powers between the executive and legislative branches.

He noted that the Senate measure includes restrictions on presidential authority, such as a requirement that spending cuts be based on "relative priorities" in the federal budget.

"However, none of these limitations alters the fundamental fact that the proposal confers on the president the power to cancel the amounts of authorized and appropriated funds, thus terminating legal rights established under constitutionally enacted legislation," Rodino said.

"While under the Constitution Congress can delegate the authority to implement laws, it cannot delegate the authority to repeal laws," he added. "This is precisely what the Senate proposal purports to do."

Rodino drew a parallel between the balanced-budget plan and legislative-veto powers that were sharply curtailed by the Supreme Court as improper shortcuts around constitutional rules for passing and implementing legislation.

While he had objections to both, Rodino said, his problems with the budget plan are "greater because the plan strikes at the core power of Congress: the power of the purse."