Sen. Gary Hart (D-Colo.) left the Democratic National Convention in July thinking he had a firm pledge from Walter F. Mondale that the two men would mount a "unity" effort to help one another pay off campaign debts.

Mondale, seeking support from his principal rival for the party's presidential nomination, had discussed the subject with Hart the final day of the convention, and Mondale aides had pledged to raise $100,000 immediately for the Colorado Democrat.

About $50,000 -- half the pledge -- materialized, due in part to a disagreement between Hart and Mondale aides over terms of the deal. And that money did not arrive at Hart headquarters until weeks after the convention.

Hart received little other assistance, despite repeated requests for help, according to former Hart campaign officials, who said Mondale aides rebuffed their efforts to set up a series of joint fund-raising events with the two Democrats.

"The senator is disappointed and frustrated," one former Hart adviser said. "He feels he worked very hard for the Mondale ticket and they haven't reciprocrated."

"The unity effort simply hasn't been seen," said Hart press secretary Beth Smith. "We're still trying, but nothing has developed yet."

She said Hart put off raising money last fall so that his efforts would not interfere with those of other Democrats. "Now we need help," she added.

Hart, who spent much of the fall campaigning for the Democratic ticket, had a debt of $4.7 million as of Dec. 1, including $1.4 million in bank loans, according to Federal Election Commission reports.

Aides said negotiations with vendors are expected to reduce that to about $3 million, a considerable amount to raise as Hart weighs running for reelection to the Senate in 1986 or mounting another presidential campaign in 1988.

Mondale has raised enough money to erase his $3 million debt, thanks in part to a series of "victory fund" events in which the former vice president split the proceeds with the Democratic Party.

Mondale received about $600,000 from these events. The rest of the money to repay his debts came from direct mail solicitations and federal matching funds, campaign officials said.

Mondale advisers differ sharply with Hart aides in their recollections about negotiations before and during the Democratic National Convention in San Francisco.

"My feeling is we more than lived up to our commitment, but we wished we could have done more," former campaign chairman James A. Johnson said. "I am certain Mondale will help out those who have campaign debts in any way that he can."

In addition to raising about $50,000 for Hart, Johnson said the Mondale campaign gave Hart lists of its donors and Mondale offered to sign a fund-raising letter for his former opponent. However, Hart aides never gave them a letter to sign, he said.

"I think they [Hart forces] are trying to blame us for their problems," another top Mondale adviser said.

Discussions about Mondale and Hart debts began well before the Democratic convention, at a time when Hart still was an active candidate for the nomination and Mondale wanted to avoid major fights with the Colorado senator on the convention floor.

Hart left these discussions believing that he and Mondale had agreed to a united debt-retirement effort. At a key session on the final day of the convention, Hart advisers said their counterparts in the Mondale campaign told them they would raise $100,000 for Hart within the week.

Mondale advisers recall a different version of the deal. Each campaign, they said, was supposed to raise $100,000 for the other.

"We raised $50,000 for them, and they didn't raise a dime for us," said Tim Finchem, Mondale's former finance director. "Tell them it is all over. They can stop picking on us."

"They mismanaged their campaign to get so far in debt, and they are mismanaging their debt-retirement effort," Finchem added.

Hart is not the only former Democratic presidential contender heavily in debt. Sen. John Glenn (D-Ohio) owes about $2.8 million and Sen. Alan Cranston (D-Calif.) about $800,000, according to aides.