The farm-credit aid announced last month by President Reagan, which was criticized by some as an election-season gambit, has been broadened by Agriculture Department officials to give more help to economically troubled farmers.

Reagan's original program would have allowed farmers in serious trouble to defer payment of 25 percent or $100,000 of their debt to the Farmers Home Administration (FmHA) for five years, without interest.

Under regulations just published, the FmHA has doubled the coverage to 25 percent or $200,000, whichever is greater. The effect is to give full benefit of the program to farmers who may owe as much as $800,000.

An agency spokesman said the change was ordered by administrator Charles W. Shuman after reviewing statistics that showed the original program would not provide as much help as the administration intended.

For other distressed borrowers, Reagan's aid program will permit the FmHA to guarantee up to 90 percent of a farmer's loan if the lender is willing to write off a minimum of 10 percent of the debt. That move, intended to help bolster farmers' cash-flow position, has drawn a mixed reception from bankers.

In a related step, Shuman also has decided to suspend issuance of FmHA's so-called pre-termination notices -- letters advising delinquent borrowers that the agency is about to move against them. The suspension will give those farmers an opportunity to enter the debt-deferral program.

Cost estimates of the reshaped deferral program are not available, but USDA officials calculated that the plan announced by Reagan could have cost the Treasury as much as $700 million.