Mayor Marion Barry's call for an "open" Democratic National Convention is seen by many observers here -- including the White House -- as an attempt by a halfhearted supporter of President Carter to hedge his bets.

Barry told the Washington Post Tuesday that he opposed a party rule change that would compel convention delegates (most of whom are committed to Carter) to vote on the first nominating ballot for the candidate whom they are pledged.

Barry's statement, which contradicts the position of the Carter campaign, created a stir among Carter supporters in the District and was seized upon by backers of Sen. Edward M. Kennedy (D-Mass.), who are pushing the open convention scenario and have said they would welcome a defection by the mayor.

Sources said Barry is known privately to prefer Kennedy.

On Wednesday, Barry issued a written statement proclaiming his loyalty to Carter. But he did'nt retract the open convention call. "I don't need a rule to bind me to vote for President Carter, and I don't think that other Carter delegates need such a rule either," he said.

Ivanhoe Donaldson, Barry's general assistant and chief political aide, denied that Barry was trying to straddle the fence between Carter and Kennedy. He said Barry believes that the issue is moot and that Carter has enough votes to win the nomination whether delegates are bound on the first ballot or not.

"He just feels that if it makes people happy to have an open convention, if it's good for party unity, then that's fine," Donaldson said. But the reaction of many observers was summed up by a White House source familiar with relationship between Barry and the Carter campaign. The source said of the Mayor: "He is hedging his bet."

The source said there has been "some discussion of Barry's statement at the White House, but added that the open convention call was "less surprising" coming from Barry than from more active Carter supporters like Connecticut Gov. Ella Grasso.

"Marion has been perhaps a reluctant supporter for some time," the White House source said. The source predicted that the relationship between Barry and the White House would not suffer much because of the statement, but added, "I would have been nice if he had told us he was going to do it."

Donaldson said Barry did inform some Carter campaign officials in advance of his intention to call for the open convention. He pointed out that Barry has also disagreed with the White House in the past on issues such as draft registration and Carter's "Rose Garden" strategy of staying in the White House and not actively campaigning in the early months of the political season.

The Carter delegates from the District said they disagree with Barry's open convention position. City Council Chairman Arrington Dixon, a Carter delegates, called on Barry to skip the convention if he could not adhere to the Carter position.

"I don't think he (Barry) has a devotion to the president," another Carter delegate said. "It's just a reflection of mediocre loyalty. This is the kind of thing that makes it impossible to count on Marion as a political ally."

"He didn't want to go (with Carter) but he did it because he thought it would help the city," the delegate said, referring to the necessary close relationship between the District Building and the White House.

Barry helped the local Carter campaign find office space, but did practically nothing else in advance of the May 6 D.C. primary, which Kennedy won handily. Barry waited until four days before the election to take the token step to taping Carter-Mondale campaign signs to his car he said late that he knew all along that Kennedy would carry the District.

Barry's statement also puts him in a somewhat different position in case the convention does break open. "If this thing really gets going, and you need some lobbying done in other delegations, who do you send" asked a Kennedy delegate. "You send Ambassador Barry. I think he wants to be there in case anything happens."