Lobbyists for the nation's major agricultural employers have mounted a massive campaign to exempt corporate farms from coverage by what they call a "nagging" federal regulation designed to protect migrant farm laborers from abuses.

The campaign already has achieved a rare lobbying coup: A majority (52) of the members of the Senate have signed a letter to Labor Secretary f. Ray Marshall suggesting that Labor Department inspectors back off from rigid enforcement of the Farm Labor Contractor Registration Act.

And two bills have been introduced in the House, one co-sponsored by 24 members, including a majority of the Virginia House delegation, that would exempt corporate farms from the legislation.

On the other side of the issue, several organizations that represent migrant workers contend that the Labor Department has been timid in enforcing the act, which requires middlemen, so-called body brokers, to register and meet certain requirements before they can hire, transport and house migrant workers.

The House manpower and housing subcommittee has called a hearing for today to investigate whether the Labor department is doing a good enough job enforcing the present law.

"The absolute irony of this situation," said Rep. Andrew Maguire (D-N.J.), a member of the subcommittee, "is that at the very time when the Department of Labor is being criticized for lack of enforcement and being investigated by the subcommittee, the agrilobby is trying to further curtail DOL's power."

Maguire said "clearly, migrant workers [form] one group in our society which needs constant support and vigilance on their behalf to prevent further exploitation."

A congressional staffer who has followed the legislative battle added that "despite puny and spotty enforcement, agribiz is lined up shoulter-to-shoulder" against a law that has resulted in "only one prison sentence" (and hundreds of fines) since its passage in 1963.

"Corporations have not been innocent of serious, substantive violations," asserts the Migrant Legal Action Program, a nonprofit group that lobbies on behalf of migrant farm-workers.

The National Food Processors Association, one of the trade groups active in the lobbying effort for the exemption said "what Congress really intended . . . was to stop abuse of migrant workers by so-called crew leaders. These people often could not be held accountable because they were transient and could not be located after an abuse was discovered. Even if found they often had no insurance or other assets to pay for any injury or wrong done to workers they had recruited."

The law requires the farm labor contractors to register, be finger-printed and get a certificate from the Department of Labor acknowledging their responsibility in complying with laws governing wages, safety and sanitation, but it exempts farmers, canners, processors and nurserymen who "personally" go out and hire migrant laborers to work on their own farms.

It is the word "personally" that agribusiness lobbyists want deleted from the law; along with a section that exempts regular farm employees who act as labor contractors "on no more than an incidental basis."

Mark S. Schacht, legislative advocate for the Migrant Legal Action Program, contends removing those two sections of the law would have the effect of freeing corporate farms from any responsiblity under the act.

Perry Ellsworth, executive director of the National Council of Agricultural Employers, who has been a prime mover in the campaign to change the law, concedes that the change would likely exempt farms that are incorporated, but said they would be covered by other state and federal laws.

Ellsworth said zealous Labor Department inspectors have been requiring farm foremen, who drive their bosses' migrant workers from temporary homes to the fields, to get certificates as farm labor contractors.

Both Ellsworth and Schacht are scheduled to appear at today's hearing, along with Craig A. Berrington, deputy assistant secretary of labor.

Berrington said last night that the department is "studying the issues" raised by the unusual" letter from the 52 senators but he noted that "historically, it has been the department's position that a corporation doesn't deserve the same exemption as Farmer Jones" from laws governing migrant workers.

Ellsworth said, "I don't deny there have been instances" where corporate farms have "fallen through" on their obligations to migrant workers. But he said that occurs in every industry, "and there are laws to cover that.'

He said he studied 5,061 citations passed out by Labor Department inspectors between mid-1977 and mid-1979 and concluded that only 5.2 percent were for "substantive violations."

Growers are "fed up" with nitpicking enforcement, Ellsworth said, and the "mountain of paperwork and red tape" posed by the far-reaching law.

Schacht contends the letter was circulated to Senate offices only after the agribusiness lobbyists failed to convince Sen. Gaylord Nelson(D-Wis) to help them.