D.C. Mayor Marion Barry and Virginia Rep. Herbert Harris came out yesterday in strong support of an "open" Democratic convention that would allow delegates to vote their political convictions on the first ballot.

In another slap at President Carter, the Fairfax County Democratic Committee narrowly voted last night to endorse an open convention because of "changed circumstances" since the presidential preference primaries held last winter and spring.

Barry and Harris -- previously among the president's most prominent supporters in the Washington area -- said in separate interviews that they are now convinced that the party's nominating convention in New York next month should be opened to candidates other than Carter and challenger Edward M. Kennedy.

"If the definition of an open convention is that those of us who are delegates are not bound by law to vote a certain way on the first ballot, then I'm in favor of an open convention," said Barry.

The mayor said he still personally favors Carter and believes the president will win renomination no matter what happens prior to the convention. Barry said he plans to advise the Carter campaign "not to get hung up" on a fight about whether the delegates are bound to the president on the first ballot.

Harris, however, leveled a blistering attack on Carter's administration and said he is "concerned" about the president's reelection chances this fall.

"Carter has failed to deal with the problems of federal workers, the retired military personnel and social security recipients," complained Harris, who only last February stood shoulder-to-shoulder with the president at the White House and effusively endorsed his reelection bid.

Asked what has changed since then, Harris said his "high hopes" of convincing the White House to take his constituents' problems more seriously had been dashed by the administration's inattention.

"Carter has lost their confidence . . . I'm more convinced the White House is alienating these people," said Harris, who faces a tough race this fall against Stanford Parris, the Republican he ousted in 1974.

The political ramifications of the Barry and Harris statements were not immediately clear.Some Virginia Democrats said they were astounded by Harris' comments, which came shortly before members of the Fairfax County Democratic Committee voted on the resolution calling for an open convention in New York.

That vote was 50 to 44, backing an open convention and urging the state's convention delegates to do the same.

One Carter supporter later dismissed the resolution as something "to toss in the circular file," but its sponsor said he hoped the vote in Fairfax would have an impact on other Virginia Democrats.

The primaries were held three to eight months ago, and it cannot be said today that they represent the will of the primary voters," argued Thomas Patton, a McLean attorney who lives in the same district where Kennedy has a home and who has supported Kennedy's nomination.

A few Carter supporters said they agreed with one speaker that Carter "needs the fresh mandate of an open convention," but most supporters of the president seemed embarrassed by the passage of the resolution and accused Kennedy backers of helping to engineer it -- which Kennedy supporters at the meeting denied.

Despite his attacks on the president, Harris made clear yesterday that he has no intention of joining a "dump Carter" movement being organized by three Dozen House Democrats. He said he did not attend a meeting of that group Friday and does not endorse its attempts to pressure Carter to release the convention delegates pledged to him.

Though the overwhelming majority of the delegates support Carter, Kennedy has a strong following among Washington area Democrats, particularly those from the District and Montgomery County. The president's poor showing in the polls against GOP nominee Ronald Reagan, coupled with recent embarrassing disclosures about Billy Carter's involvements with Libya, have fueled hopes by Kennedy supporters and others that the convention will nominate an alternative to Carter.

"I'm getting a tremendously positive response -- people are saying "Thank goodness, some people are looking for an alternative,'" Montgomery County Rep. Michael K. Barnes, a spokesman for the "dump Carter" Democrats, said yesterday.

Barnes said he had a stack of letters and telegrams "three inches high" on his congressional desk, all indicating "a fantastic outpouring of support" for efforts to find alternatives to Carter and Kennedy. He said he remains uncommitted through other Democrats have taken a talking up the possible candidacies of Vice President Walter Mondale, Secretary of State Edmund Muskie and Sen. Henry M. Jackson of Washington.

Harris said yesterday that he think Mondale would be the best replacement for Carter to head the ticket.

Despite the "dump Carter" movement by the congressional group and the speculation over alternative candidates, District, Maryland and Virginia officials said yesterday they have not found any local delegates who appear ready to abandon their allegiance to the president.

"There's no way I'm interested in switching," said attorney John Schell, a Carter delegate from McLean who campaigned unsuccessfully for the Virginia Democratic nomination for state attorney general in 1976.

Schell said any attempt to oust Carter without an alternative nominee would be "bad politics," but he conceded that there were reason to worry about the party's prospects in November.

"You'd have to be Scarlett O'Hara not to be concerned," said Schell. "In Northern Virginia, we've got two very good congressmen (Harris and Rep. Joseph L. Fisher) who could be in trouble."

But Schell added that those who might be nervous about having Carter head the ticket would not support an open convention "if Kennedy is the only alternative."

In the District, Council member John Ray said the renewed talk of an open convention was not having any impact on Carter delegates.

"We will be working on them," said Ray, who chairs a 19-member delegation dominated by 12 Kennedy suporters. He was at Kennedy's home in Hyannisport, Mass. Last weekend mapping convention strategy with other advisers.

Ray said he was not surprised that Carter supporters in Virginia, Maryland and the District were holding fast to their candidate, but he said the situation would be different in the delegations from industrial states.

"We've got a lot of black delegates now pledged to Carter who frankly like a lot of what Kennedy is saying and if the convention is open it will be very difficult for a third candidate to come in and get the nomination." Ray said.

An aide to Rep. Fisher and yesterday that the Virginia congressman supports the president and believes the presidential primaries had guaranteed an open selection process.

Last night, Kennedy hosted a strategy session for around 100 delegates, alternates and supporters from Maryland, Virginia and the District at his rambling McLean estate. Dicussion centered on the open convention scenario, with Kennedy telling reporters he expects pressure from the public and political leaders to force Carter to release his delegates.

D.C. Del. Walter E. Fauntroy, who has sung "The Impossible Dream" at a number of campaign appearances for Kennedy this year, referred to the senator's effort last night as "the now-possible dream."