Nine pharmaceutical companies issued a joint pledge on Tuesday that they would “stand with science” and not put forward a vaccine until it had been thoroughly vetted for safety and efficacy.

The companies did not rule out seeking an emergency authorization of their vaccines, but promised that any potential coronavirus vaccine would be decided based on “large, high quality clinical trials” and that the companies would follow guidance from regulatory agencies like the Food and Drug Administration.

“We believe this pledge will help ensure public confidence in the rigorous scientific and regulatory process by which Covid-19 vaccines are evaluated and may ultimately be approved,” the companies said.

President Trump has repeatedly claimed in recent weeks that a vaccine could be available before Election Day — Nov. 3 — heightening fears that his administration is politicizing the race to develop a vaccine and potentially undermining public trust in any vaccine approved.

“We’ll have the vaccine soon, maybe before a special date,” the president said on Monday. “You know what date I’m talking about.”

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The move was welcomed by some researchers who said that the statement could increase public confidence in a coronavirus vaccine at a time when skepticism was running high. “There’s absolutely a desperate need for this vaccine,” said Dr. Judith Feinberg, the vice chairwoman for research in medicine at West Virginia University in Morgantown. “I love the fact that the nine big vaccine manufacturers today said they would not do anything premature — I think there’s enormous pressure to do something premature.”

Three of the companies that signed the pledge are testing their candidate vaccines in late-stage clinical trials in the United States: Pfizer, Moderna and AstraZeneca.

Pfizer has said repeatedly over the past week that it could apply to the F.D.A. for emergency approval as early as October. On Tuesday, its chief executive, Dr. Albert Bourla, predicted in an interview on the “Today” show on NBC that the company would have an answer about whether its vaccine worked by the end of October, but acknowledged that did not mean its vaccine would be available to the public by then.

Moderna and AstraZeneca have been less specific, saying only they hope to have a vaccine by the end of the year. Last week, Moderna’s chief executive said the company was slightly slowing its enrollment in order to include more people from groups that had been most affected by Covid-19.

Pfizer and Moderna are each close to fully enrolling the 30,000 participants in each of their trials, with some analysts predicting they will be finished within the next two weeks. AstraZeneca is further behind in its U.S. trials, having begun enrollment on Aug. 31.

Federal officials have been pushing back against Mr. Trump’s enthusiastic predictions. Late last week, Moncef Slaoui, the top scientist on Operation Warp Speed, the federal effort to quickly bring a vaccine to market, warned in an interview with National Public Radio that the chance of successful vaccine results by October was “very, very low.”

And on Tuesday, Dr. Anthony S. Fauci, the nation’s top infectious disease expert, said he believed that researchers would know whether the Moderna and Pfizer vaccines were effective by “November or December.”

In a statement on Tuesday, Dr. Slaoui said the goal of Operation Warp Speed was “to ensure that no technical, logistic or financial hurdles hinder vaccine development or deployment without curtailing the critical steps required by sound science and regulatory standards.” He added that the pledge “reiterates the position of Operation Warp Speed, that this project is driven by science and that any vaccine must meet the gold standard of the Food and Drug Administration.”

Drug companies have had to carefully navigate the political landscape. A successful vaccine could help restore the industry’s battered image and offer an end to the pandemic. But rushing a vaccine to market that winds up causing serious side effects — or simply does not work — could do catastrophic damage to their reputations.

In the nine companies’ statement on Tuesday, they did not mention Mr. Trump, saying only that they have “a united commitment to uphold the integrity of the scientific process.”

The other six companies that signed the pledge were BioNTech, which is developing the vaccine in partnership with Pfizer, GlaxoSmithKline, Johnson & Johnson, Merck, Novavax and Sanofi.