The vice chairman of the Legal Services Corp., John N. Erlenborn, resigned abruptly this week after a farmers group opposed to him directed an intense lobbying campaign at his law firm and the firm's clients, accusing him of conflict of interest.

Erlenborn, a former Republican representative from Illinois named to the board by President Bush as a recess appointee last November, informed the White House of his resignation following the lobbying by the American Farm Bureau Federation. The issue was first reported last night by CBS News.

The Farm Bureau is opposed to Legal Services lawyers' work on behalf of migrant workers and has been in the forefront of efforts to "reform" Legal Services, long a favorite conservative target, by imposing restrictions on its lawyers' activities.

Erlenborn, a House member from 1965 to 1985, is now a partner with the Washington law firm of Seyfarth, Shaw, Fairweather & Geraldson. At the time of his appointment, Libby Whitley, the Farm Bureau's assistant director for national affairs, described him as "someone we can work with."

But in an April 6 memorandum to two Seyfarth, Shaw clients, the New England Apple Council and Agricultural Producers Inc., the Farm Bureau said it had "become increasingly concerned" about Erlenborn.

"It is important to alert both your organizations about what are perceived as early indications that the former congressman, who is a senior partner in the law firm with which you do a great deal of business, is working against agricultural interests," the memo said.

On April 27, in a letter to two Seyfarth, Shaw partners, Whitley said Erlenborn's participation in a number of activities as a Legal Services board member "would conflict with grower interests."

The next month, Whitley wrote to the firm's managing partner, Richard C. Johnson, complaining about Erlenborn's activities opposing senior Legal Services Corp. official James Wootton.

"Farm Bureau regards his recent opposition to Jim Wootton, and thus his efforts to undermine the reform effort, as hostile to our position, and contrary to the interests of your clients," Whitley wrote.

In an interview last night, Sen. Warren B. Rudman (R-N.H.), a vehement opponent of the reform movement, angrily blasted the Farm Bureau's actions as "outrageous" and "one of the least subtle attempts at blackmail that I've seen."

Rudman, who championed Erlenborn's appointment to the board, vowed that the Farm Bureau "will pay for this. . . . Even people who don't like Legal Services, they don't like a former member of Congress being treated that way."

Erlenborn said he had offered to refrain from voting on particular issues but eventually decided to resign because his firm has a large labor law and agricultural practice. "I didn't want to put my partners through this kind of controversy," he said.

In a statement, the firm said it supported Erlenborn's decision to resign and that the "perceived conflicts . . . were not foreseen at the time" of Erlenborn's appointment. Erlenborn said his membership was cleared with firm clients, including one who now objects.

In a May 25 letter to Agriculture Secretary Clayton K. Yeutter, Whitley, who could not be reached last night, said it was "our professional responsibility" to alert the firm and its clients to Erlenborn's "apparent conflict." She said the group had not attempted to "pressure the firm" or "coerce" Erlenborn into supporting its position.

Erlenborn's service on the board "was a totally improper arrangement," and his resignation "speaks for itself," said Peter T. Flaherty, who cochairs with Whitley the Legal Services Reform Coalition.