Officials of the farmer-owned Farm Credit System (FCS), which holds about a third of the nation's $215 billion agricultural debt, urged a House subcommittee yesterday to support a massive federal bailout proposal to keep the system afloat.

Without federal assistance, they warned, the system could collapse within two years because of rapidly rising loan losses and abandonment by investors who buy the bonds that finance the FCS' 37 banks and more than 700 lending associations.

Yesterday's hearing before the House Agriculture subcommittee on rural credit was the first round in what promises to be a long and heated debate over ways to deal with the deepening problems of farm credit in general and the FCS in particular.

Commercial banking interests, for example, were critical yesterday of proposals for federal subsidization of the farmer-owned system and urged that it be given only on the condition that there be more federal oversight and protection against unfair competition.

Still to be heard from is the Reagan administration, which is considering an aid package that would include a Treasury line of credit for the FCS, officials said. The Wall Street Journal said the proposal was presented to the president, but officials said Reagan had not reached a decision.

Agriculture Department credit official Frank W. Naylor Jr. was to have outlined the administration's position before the subcommittee yesterday, but he postponed his appearance until today, saying he might have more up-to-date information.

Naylor also is to testify today before the Senate Agriculture Committee, which is beginning its own hearings on the credit issue.

Donald E. Wilkinson, chief executive of the quasi-federal Farm Credit Administration, which oversees the lending system, told the subcommittee yesterday that "we have no indication the administration supports the $5 billion line of credit."

Gene Swackhamer of Baltimore, representing the system, said that a default by the FCS would have an impact on financial markets and the national economy that would be "more on the order of a tidal wave than a ripple."

The proposal by the Farm Credit Council, which represents the various branches of the FCS, called for Treasury loans of up to $6 billion to help the banking network cope with its mounting troubles and restore shaken confidence of farmers and investors.

In return for the federal props, Swackhamer said the FCS would agree to carry out major internal reforms and it would accept more muscle for the Farm Credit Administration to put it on a par with other federal banking regulators.

The FCA's Wilkinson agreed with the council's somber assessment of the impact of a system failure on the national economy.

But Wilkinson recommended that Washington set up a $5 billion backup line of credit that the system could draw upon as needed once the FCS has "adequately committed" it own resources to solving its problems.

Wilkinson told the subcommittee that the federal aid would be needed within the next 18 to 24 months, but that failure to act soon will cost farmer members of the FCS additional millions of dollars and intensify pressures to liquidate the weakest parts of the system.

Subcommittee members, although generally sympathetic to aiding the FCS, took turns lambasting Wilkinson and the FCA for allowing the situation to reach a crisis level before turning to Congress and the administration for help.

"It really gets to me," complained Rep. E. Thomas Coleman (R-Mo.). "We haven't seen this much interest in $5 billion to help farmers before . . . . Let's recognize that you are asking us to buy a life insurance policy for somebody who has died."

"The time to be nice is over," said Rep. Larry J. Hopkins (R-Ky.). "Where in the hell have you been? . . . How in the world are you going to solve this problem with $5 billion? I don't think it will work."

Said Rep. Thomas A. Daschle (D-S.D.), picking up a frequently used allusion to the Titanic: "All segments of agriculture are on the Titanic. Now, with the Titanic sinking, the FCS is attempting to swim to safety. Your statement doesn't even give lip service to farm income. If you can't put your finger on the source of the problem we are facing now . . . ."