Gary Hart's 1984 creditors suffered another legal setback yesterday, when the Federal Election Commission said it had no authority to tie up Hart's federal matching funds for 1988.

FEC General Counsel Lawrence M. Noble wrote two Hart vendors, Xerox Corp. and Semper-Moser Associates of Venice, Calif., that the Treasury Department, not the FEC, controlled the $100,000 in matching funds Hart was eligible to receive. A judgment against Treasury is barred by sovereign immunity, he added.

Last week a federal magistrate in Denver ruled that the two Hart campaigns were separate corporate identities, blocking a Semper-Moser effort to get another $100,000 raised privately for the 1988 race.

Lawyers for the two businesses said yesterday they are running out of legal options, but are hopeful that Hart's renewed 1988 campaign will cooperate in finding a way to pay off more than $1 million in debt from the 1984 campaign.

Warren Rosenfeld, whose client Xerox is owed about $10,000, said he saw nothing in the FEC letter that would bar Hart from asking the FEC to send the matching funds to his 1984 campaign committee. But an FEC spokesman said that issue hasn't been resolved because Hart hasn't asked for an advisory opinion on it.

James Turner, lawyer for Semper-Moser, a company owed $162,000 for buying radio ads, said the FEC position was expected. He added that he is still negotiating with Hart's new campaign attorney on several possible ways to pay the debt.