Senate Republican leaders, facing mounting difficulties on two crucial fronts, yesterday resorted to delaying tactics to fend off possible defeat on farm-credit legislation and acknowledged that their deficit-reduction effort is in serious trouble.

With Democrats and dissident farm-state Republicans within striking distance of passing farm-credit legislation opposed by the Reagan administration, Majority Leader Robert J. Dole (R-Kan.) put off a vote on the issue and hinted at further delaying tactics if it appears the measure will pass.

Less than a week ago, it was Dole who fretted at the farm-state coalition for holding up confirmation of Edwin Meese III as attorney general to force action on farm legislation. Dole had been confident of his votes for Meese, who was confirmed by a 2-to-1 majority Saturday. He acknowledged yesterday that he was not sure he had the votes to pigeonhole the farm legislation.

The farm bill at issue yesterday would go considerably beyond steps taken by the administration last week by further easing credit terms, providing $100 million for federally subsidized interest payments and increasing funds available for loan guarantees by $1.8 billion.

A controversial provision that could result in government assumption of bad loans, with the banks bearing little or no share of the cost, was expected to be shelved, increasing prospects for Republican support of the measure.

Dole was pressing hard to dissuade Republicans from supporting the measure, and the delay appeared aimed at giving the administration time to step up its lobbying against it.

Agriculture Secretary John R. Block said yesterday in a letter to Minority Leader Robert C. Byrd (D-W.Va.) that he would recommend a veto if the legislation is approved.

One problem is that the Senate farm legislation is tied to a bill providing famine relief for starving Africans, which Republicans would not like to see President Reagan veto.

Pulling Republicans in the opposite direction were hundreds of farmers and farm-state public officials, including nearly the entire South Dakota state legislature, who milled around the Capitol lobbying for the relief.

Concerning the budget, Senate Budget Committee Chairman Pete V. Domenici (R-N.M.) acknowledged after a Republican discussion of deficit-reduction prospects that "the chances of getting anything are not very good . . . . We've got a long way to go on the whole thing."

Dole expressed more optimism but conceded that the effort was made "more difficult" by news from Domenici that it will take $64 billion in spending cuts next year, half again as much as estimated only two months ago, to meet the GOP's target of maximum $100 billion deficits by fiscal 1988.

Domenici presented an outline for achieving the goal that included defense and Social Security freezes along with nearly all of the drastic domestic spending cuts that Reagan has proposed.

The outline, called "The Promised Land" by the Budget Committee staff, was a "pretty scary piece of paper," a GOP senator said as he emerged from the closed-door session. A staffer who attended the meeting described the senators as "stunned."

Another complicating factor was a solid phalanx of opposition to a defense freeze from Republicans on the Armed Services Committee. They said they would accept no less than a 4 percent after-inflation increase for defense, which would save $11 billion next year as opposed to $20 billion savings from a freeze.