The Republican-led Senate Agriculture Committee yesterday handed the Reagan administration a new tool to curb federal farm spending, a freeze on direct subsidy payments, but Democrats hinted that they may try to talk it to death on the Senate floor.

The committee voted 10 to 7 to give Secretary of Agriculture John R. Block the authority to freeze 1984 and 1985 target prices at current levels, which Block contended would save $369 million on a $3 billion bill next year and more than $1 billion in fiscal 1985.

Block termed the committee move "a very significant and important step in making adjustments in the farm program . . . but it is just a first step."

With farm-program costs expected to pass a record $21 billion this year, the administration has put heavy pressure on the House and Senate Agriculture committees to limit the subsidies paid to wheat, corn, feed grain, rice and cotton farmers

Chairman Jesse Helms (R-N.C.) said he hoped to get the measure before the Senate before the July 4 recess..

Sen. John Melcher (D-Mont.), defeated on four moves to alter or kill the freeze in committee, complained that the pressure campaign was "the biggest use of sheer power of the executive branch that I've witnessed in 14 years on the House and Senate agriculture committees."

Melcher and Sen. David L. Boren (D-Okla.) said that they will subject the target price freeze bill to a probing discussion on the Senate floor. Boren talked of "lengthy proceedings."

And Sen. Mark Andrews (R-N.D.) warned that a target price freeze would lead Block to lower the price-support loan level for wheat next year--a possibility that administration officials would not deny flatly yesterday.

"I am getting damned sick and tired of selling our wheat to the Soviet Union and the People's Republic of China at less than the cost of production," Andrews said. "You ought to be raising the price support. This is just one more kick in the head to wheat farmers."

The winning vote on the target freeze apparently was assured with the addition of language, opposed by the administration, directing the spending of an extra $600 million on agricultural export development over the next two years.

Sens. Roger W. Jepsen (R-Iowa) and Walter D. Huddleston (D-Ky.), sponsors of the amendment, argued that it would assure farmers that while target prices were being frozen, the government would be moving to expand farm sales overseas--a situation that Block insisted was occurring already.

Although grain-state senators were reluctant to tamper with the target price formula that assures automatic yearly increases, the administration put them in a vise, with Block warning that White House support of a committee-approved revision in the dairy price-support program is contingent on the target freeze.

But the committee passed a tobacco price-support freeze and reductions in the dairy support price--both moves calculated to align oversupply with demand--in a separate package in an effort to isolate them from the more controversial and possibly more endangered target price freeze.

The House Agriculture Committee has adopted dairy and tobacco program changes nearly identical to the Senate's, but has scheduled no action on the target freeze proposal although it was debated at length last month.