James H. McMullin, a prominent Northern Virginia real estate broker and adviser to local political leaders, defended himself before the Virginia Real Estate Commission yesterday against charges of "substantial misrepresentation" and failure to disclose his personal financial interest in an Arlington land deal.

McMullin, 60, president of Real Estate Services Inc., a brokerage firm, and chairman of Arlington's Economic Development Commission, denied any wrongdoing and said, "I had no reason to disguise or withhold anything."

McMullin is a former president of the Virginia Association of Realtors and is a close adviser to the Republican majority that controls the Arlington County Board.

He is accused of having secretly arranged to buy, through an associate, a house in Arlington's Ballston section while his firm was acting as a broker for the sale.

Samuel G. Abramson, a longtime employee and officer of Real Estate Services, also has been charged with misrepresentation and acting for both the buyer and seller in the transaction without knowledge of the parties.

Abramson has also denied the charges against him.

The real estate commission has power to impose penalties ranging from a reprimand to revocation of the men's brokerage licenses.

The charges, which were the subject of yesterday's day-long hearing in Arlington before the five-member commission, grew out of the aborted 1977 sale by Roger and Mildred Marquis of their home at 1034 N. Quincy St.

The Marquis' modest stucco bungalow is adjacent to a high-rise apartment building partly owned by McMullin and built in 1964 despite the protests of the Marquises. The property, located across from Arlington's Central Library, is a quarter mile from the Ballston Metro station.

Marquis, 68, a retired Justice Department official, testified yesterday that he and his wife decided to sell their home in 1977 because both had been seriously ill and were planning to move to Florida.

Because the couple was then wintering in Florida, they signed over the power of attorney over the home they had lived in for 25 years to their nephew, Albert Pupo, so that he could arrange a sale.

Marquis, who had bitterly opposed the rezoning of his neighborhood to permit construction of McMullin's apartment building in the mid 1960s, said he told his nephew that he would never sell to McMullin or anyone associated with the apartment construction.

"I was adamant that I was going to resist any attempt [by McMullin] to secure my property," Marquis testified. "I thought he was taking advantage of other property owners who didn't know the value of their property."

Pupo testified that he listed the home for $65,000 with another brokerage firm, which was a member of the multiple listing service to which McMullin's firm also belongs.

According to the documents filed with the commission, when McMullin learned the property was for sale, he contacted Frank Kaufman, a wealthy Arlington businessman he knew and asked Kaufman to make a $63,000 offer on the house. Abramson, the documents charge, was to act as a cobroker for the sale and Real Estate Services would have split the commission with another company had the deal been consummated.

The sales contract specified that the property would be conveyed to Kaufman "or assigns." McMullin is accused of knowing that the property would be assigned to a dummy corporation, Edjim, of which McMullin is sole officer and stockholder, and failing to disclose that to the Marquises.

Both Marquis and Pupo said they did not know that the contract would be assigned to someonw else or of McMullin's involvement. "I just wanted to get rid of the property at a fair price," said Marquis, who said he assumed that Kaufman was the purchaser.

The sale was never consumated. Marquis refused to sign the settlement contract because he said he felt he was being pressured to move too fast.

Marquis said he learned of McMullin's involvement when Edjim Corp. sued the couple for breach of contract. That suit was dismissed in 1978 and the couple then filed a complaint with the Real Estate Commission.

Abramson contradicted Marquis' testimony saying he told Mildred Marquis that the apartment owners were the prospective buyers when she asked him if that would be the case. Abramson said he did not know of Edjim's involvement until the settlement date nor was he aware of McMullin's interest in that corporation.

McMullin denied that he would have benefited from the sale. He testified he had an oral agreement with another longtime business partner and co-owner of the apartment building that Edjim would only act as a trustee until the partner decided how to dispose of the property."

"I had no reason to think it would make any difference to the Marquises" if they knew Real Estate Services was the cobroker, said McMullin who testified he never told the couple of his involvement with Edjim.

The commission is expected to make a decision within a month, according to commission director David Seitz.