At least two House Democratic leaders have joined in supporting an alternative to the balanced-budget constitutional amendment that President Reagan backs -- a simple bill that encourages but stops short of requiring an end to deficits.

Their effort, which is also backed by some Republicans, came to light yesterday as the Democratic House and Republican Senate continued to jockey for advantage in shaping the huge stop-gap spending bill that Congress must pass by the start of the new fiscal year Oct. 1.

With time drawing short for putting together this so-called continuing resolution, the Senate fears it will be shut out of bargaining over spending levels unless it takes at least some action on money bills before the House, which normally acts first on appropriations.

The Senate's Appropriations subcommittee on agriculture yesterday approved $27.4 billion for farm and nutrition programs. That level is within the congressional budget but about $2 billion more than the administration asked for.

The Senate plans similar committee action on at least half the 13 regular appropriations bills, including the one for defense, which has yet to win even subcommittee approval on the House side.

Senate leadership sources say they fear President Reagan may veto the continuing resolution, especially if it includes less money for defense than he wants.

So, the sources said, the Senate may seek to pass a separate defense spending bill, so as to have its amount on the bargaining table when it comes time to work out the continuing resolution.

The House plans to approve a housing and independent agencies money bill this week. Only one other appropriations bill, involving military construction, has been passed by the House.

The proposed balanced-budget legislation has been circulating quietly in the House since last month and surfaced in a hearing on budget reforms, where House Budget Committee Chairman James R. Jones (D-Okla.), who is spearheading the effort along with House Democratic Whip Thomas S. Foley (D-Wash.), described it as an alternative to a constitutional amendment.

"You should not go headlong into a constitutional amendment based on a theory that economists disagree on," said Jones, who has prepared a draft of the legislation and may introduce it or a variation of it before Congress quits for the elections next month, according to aides.

The proposed constitutional amendment, which has passed the Senate but is bottled up in the House Judiciary Committee, requires a balanced budget in peacetime except when three-fifths of both houses agree to permit deficits. A petition to force the amendment to the floor for a vote is stalled 14 votes short of the required 218 signatures. But an effort is under way, supported by Reagan, to get the measure to the House floor this year.

The Jones proposal contains some of the same features as the amendment, including a three-fifths vote for budget-busting, but it would not be part of the Constitution and it contains broader escape clauses than the amendment does.

It would require both the president and the budget committees of the two houses to submit balanced budgets each year, but they could also submit unbalanced budgets if they decide that a deficit is necessary.

A three-fifths vote of both the House and Senate would be required, instead of the present simple majority vote, to waive spending ceilings or revenue floors contained in the congressional budget.

To ratchet the budget down to the point where balance is possible, the Jones proposal would require gradual reductions in federal spending as a percentage of the nation's total economy.

The proposal received qualified support yesterday from the cochairmen of the Committee for a Responsible Federal Budget: former chairman of the House Budget Committee Robert N. Giaimo (D-Conn.) and former senator Henry Bellmon (R-Okla.), who was ranking minority member of the Senate Budget Committee.

But reservations were expressed by Senate Budget Chairman Pete V. Domenici (R-N.M.) and by House Government Operations Committee Chairman Jack Brooks (D-Tex.).