When conservative activist James McClellan was awarded $337,000 by the Legal Services Corp. last year, critics questioned whether the grant was based mainly on his ideology.

Those questions have multiplied as McClellan has fallen far behind in his federally funded work while embarking on a political career. He is running as a Republican for the House of Delegates in southern Virginia, resulting in charges by his opponent and others that the federal grant may be subsidizing his campaign.

A congressional inquiry is now examining the central question about McClellan: Can one man run a nationally known conservative think tank, a federally funded service for the poor and a partisan campaign out of the same Cumberland farmhouse without blurring the lines among them?

The criticism is particularly ironic because McClellan, a former aide to Sen. Jesse Helms (R-N.C.), has long complained about liberal groups misusing federal programs for political ends.

McClellan dismisses the allegations as "political harassment." He said his fledgling Constitutional Law Center is "very objective" and completely separate from his political activities, and blamed its slow progress on funding delays.

"It takes awhile to start a new organization from scratch," McClellan said. "We're way down in the country, and things move slower down here."

McClellan said the center has given limited help to legal aid lawyers because "there was sort of a boycott against us. Legal services groups were very upset that an organization funded by the Legal Services Corp. is run by people of the conservative persuasion. There seems to be a presumption in the liberal world . . . that conservatives couldn't possibly be interested in helping the poor."

Rep. Robert W. Kastenmeier (D-Wis.), chairman of a House Judiciary subcommittee, is investigating the grant and has asked the General Accounting Office for a report before the November election. People for the American Way, a nonprofit liberal lobby, plans to ask the Legal Services Corp. this week to conduct its own probe.

The corporation bars any "staff attorney" who receives at least half his income from the agency from seeking political office. Legal Services officials said at first that this did not apply to McClellan, a law school graduate, because he does not belong to any state bar and therefore is not an attorney.

The corporation now says McClellan is exempt because he receives less than a quarter of his income from Legal Services. McClellan said he accepts less than the $45,000 salary listed under the grant, but refused to reveal the amount. He also disputed charges that he uses a car and other equipment leased under the grant for his campaign.

Watkins Abbitt Jr., McClellan's Democratic opponent for the Virginia legislature, said he questions whether McClellan is "using taxpayers' money to support his run for the House of Delegates. I think Mr. McClellan has got something to hide and he doesn't want it brought out in the open."

McClellan is a former educator who wields substantial influence with the Reagan administration. He is best known as head of the Center for Judicial Studies, a think tank that crusades against liberal legal theory through an increasingly visible magazine called Benchmark.

McClellan said the Center for Judicial Studies has been largely inactive this year and relinquished its two buildings on his Cumberland property to the Legal Services organization. If his one-year Legal Services grant is not renewed, he said, the Center for Judicial Studies will move back into those quarters.

McClellan's Legal Services grant was one of three awarded to conservative groups on the last day of fiscal '84 that drew criticism because of irregular procedures. Legal Services officials have cut off funds to another of the grantees in a dispute over lack of progress.

Legal Services Vice President Charles Jarvis said McClellan's group has an extension to finish its work by Dec. 31. "We have attempted to hold them accountable, rigorously, for this one-year grant," he said. Jarvis said some of the center's work "should have been done much earlier. They are probably, in the last month, beginning to bring their work up to the schedule it should be at."

But Bucky Askew, a lawyer with the National Legal Aid and Defender Association, said McClellan's center "has not made itself known. I've seen no product come out of that center. There has been no evidence of their existence."

The center has published only two of six required newsletters and has finally scheduled a conference for legal aid lawyers. The session in Washington next month will examine the problem of groups using federal funds earmarked for the poor for political activities.

That is precisely what McClellan's critics accuse him of doing. The federally funded center has nearly completed the first of two litigation manuals, which deals with religious freedom in schools. McClellan had wanted to publish a second on how gun control affects the poor, but Legal Services officials rejected the idea.

"The center has not rendered any technical assistance, nor has it become involved in any litigation," Legal Services official Michael J. Murphy said in an internal memo in June. He said that "no Legal Services field programs have utilized the services of the center to date . . . . I am also uncertain as to how the center is allocating employe time and expenses between the Center for Judicial Studies and itself."