President Carter's political strategists mounted a counterattack yesterday on the movement for an "open" Democratic National Convention in New York City next month and made clear that they are in no mood to back down on the issue to promote party unity.

Pleading for a chance to "put into perspective one of the great PR coups of all time," Robert S. Strauss, the Carter campaign committee chairman, accused proponents of the open convention idea of "mislabeling." He said their proposal would actually close the convention and turn back the clock to the days when "politicians and power brokers" dictated the choice of a presidential nominee.

The mounting number of calls for the president voluntarily to release his delegates from their commitments "sounds logical," Strauss said. But to do so, he added, would represent a turning away from more than a decade of effort to "reform" party rules to assure that the convention reflects the outcome of presidential primaries and caucuses.

Strauss said the Carter campaign has found "no indication of any cause for any alarm" about the open convention flap. But he was concerned enough about it to call yesterday's news conference, during which he confessed to a certain degree of irritation about the face that the idea has been given "such a free ride" in the press.

The Carter campaign's hope of stunting the growth of the open convention movement received a boost yesterday from three senior Democratic figures who suggested that they do not expect it to take hold at the convention.

House Speaker Thomas P. (Tip) O'Neill Jr. (Mass.) said that as the convention's permanent chairman he had to be neutral on the issue. But he added that "my honest opinion is that it wouldn't make any difference" however the dispute goes. After "a lot of noise," Carter will be renominated, O'Neill predicted.

Rep. Morris K. Udall (Ariz.), sometimes mentioned as an alternative to the president and Sen. Edward M. Kennedy (Mass.), took himself out of the picture with a wisecrack."If nominated I will run to the Mexican border. If elected, I will fight extradition," he said.

Udall said that a Harris poll last week showing Carter at a historic low in public approval had increased the nervousness among jittery congressional Democrats worried about their own reelections. But now "that's calmed down," he said. "I don't see a tide starting and everyone running to the lifeboats."

Secretary of State Edmund S. Muskie, also mentioned as a possible alternative to Carter and Kennedy, angrily reaffirmed his loyalty to the president. Clearly irritated that an earlier statement on the subject was interpreted as leaving room for him to accept a draft for the nomination, Muskie said. "I've got a commitment to do this job . . . I don't intend to be part of any effort which could dilute that point. The same is true of my commitment to President Carter."

The "open convention" forces received another boost with the release of a new poll by the Mervin D. Field organization showing the president running last in California in a three-way race with Republican nominee Ronald Reagan and independent candidate John B. Anderson.

The statewide poll gave Reagan 51 percent of the California vote, Anderson 23 percent and Carter 20 percent. Kennedy ran slightly stronger than Carter in a similar three-way race, getting 24 percent compared with Reagan's 49 percent and Anderson's 23 percent.

The "open convention" group has scheduled a news conference today to announce that prominent Washington lawyer Edward Bennett Williams will become chairman and chief spokesman, according to Rep. Michael Barnes (Md.), who has been leading the effort.

Meanwhile, Kennedy, Carter's only current challenger and the potential beneficiary of the "open convention" movement, declared flatly yesterday "there will be an open convention."

On a campaign trip to Philadelphia, Kennedy said there will be "such pressure among the working members of the Democratic Party that [Carter] will have to yield to it."

The Massachusetts senator said he would continue his push for the nomination right up to the convention balloting, but he also praised Muskie, Sen. Henry M. Jackson (Wash.) and Vice President Mondale and said "if they want to be contestants at the convention, I would welcome it."

Kennedy, who has stepped up his campaign activity noticeably in the past week, went to Pennsylvania and New Jersey to deliver pep talks to his delegates. In Philadelphia last evening he greeted rush hour commuters outside City Hall.

At the session with Kennedy's New Jersey delegates, their leader, Rep. James Howard, a veteran congressman, said there were only minimal signs in the state of discontent among Carter-pledged delegates.

Howard said "maybe four or five" of the Carter delegates in New Jersey were wavering on the "open convention" question. He said he knew of no Carter delegate who was likely to desert the president on the nomination.

The "open convention" dispute centers on a rule, to be proposed to the convention by the Carter-dominated rules committee, that would require delegates to vote on the first convention ballot for the presidential candidate they were elected to represent, Rebellious delegates could be replaced by loyal alternates under the rule.

The "open convention" movement is heavily sprinkled with Kennedy supporters. The president now has more than enough delegates to win the nomination, and Kennedy bases his slim hope on winning the rules fight, hoping such an upset would set off a stampede of delegates to him.

At his news conference, Strauss took a swipe at the Kennedy backers who are supporting the "open convention" movement.

"Losers always need confusion, and losers need changes if they are to turn things around," he said.

The Carter campaign chairman argued that the convention will be "open" because none of the delegates will be bound to vote for the rule binding them on the first ballot to the candidate they were elected to support. bHe said the delegates are free to reject the rule but should not because they represent 19 million Democrats who participated in the primary and caucus process.

"There has never been a convention in the history of the nation that has been as open as this one because 19 million people will be in the hall," he said.