Top Carter administration officials launched a swarming attack on Ronald Reagan yesterday, stinging him on issues ranging from the environment to military preparedness and his "campaign conversion to support for labor-union causes.

The assault began here just hours after President Carter acknowledged that his personal attacks on Reagan had backfired and that he plans to tone down his rhetoric in the final 3 1/2 weeks of the campaign.

Campaigning in the South, Carter lowered the stridency of his rehtoric but continued to body-punch with attacks on the Republican's stands on nuclear arms control and his accusation that the United States has become militarily impotent.

The drumbeat from Carter surrogates began here in the morning with the top White House environmental adviser, Gus Speth, charging that Reagan is "ignorant of the human suffering and economic losses caused by pollution."

By midday, in El Paso, Defense Secretary Harold Brown was attempting to diffuse Reagan charges that the nation is ill-prepared militarily. Brown said U.S. armed forces are "ready to go to war -- if need be," and that charges of military weakness are "strongly misleading."

In the afternoon, Labor Secretary Ray Marshall accused Reagan of playing a "shell game" in a "friend-of-the-worker charade" on the campaign trail in an attempt to disguise a "relentlessly shameful" labor record.

The attacks appeared to be part of a long-planned Carter campaign strategy to attempt to keep the front-running Republican off balance and responding to charges from a battery of Cabinet officers and other administration officials who will fan out across the country between now and Nov. 4.

In St. Louis, Reagan felt required to respond to the early-day charges by Speth. Reagan said, "I am an environmentalist," but then renewed his charges that the Environmental Protection Agency tends to "insist on unreasonable and many times untried standards."

Speth opened the attack by calling Reagan's environmental views "strange and bewildering."

Speth said he was perplexed by Reagan's campaign-trail statement that air pollution is "substantially controlled" while Los Angeles smog has placed that city on its worst smog alert in history.

Speth also said that Reagan's comparison of pollution from Mount St. Helens with automobile exhaust showed that "Gov. Reagan doesn't understand the nature and causes of pollution."

Reagan said earlier this week that the volcano had spewed more sulfur dioxide into the air than all the auto exhaust in the past 10 years. But Speth said sulfur dioxide is not a major problem in auto exhaust and autos are not regulated against it. Electric power plants are the major environmental offender with sulfur dioxide, Speth said, and over the past decade they have produced a volume of the pollutant 500 times greater than the volcano.

In El Paso, Brown said criticism of American mlitary preparedness does not take into consideration evidence that two-thirds of the Soviet army divisions would be listed as "not combat-ready" if rating standards were applied equally to Moscow's forces.

Meanwhile, Marshall pointedly ridiculed what he called Reagan's attempt to make "a Saturday-night conversion" and portray himself as a long-time friend of American labor.

"It is clear now that Ronald Reagan will do anything to get elected, including . . . disguising his record and misleading workers," Marshall said. "Republicans must think workers are awfully dumb or simple-minded to buy all that stuff they're putting out.

"I've had to fight almost every day in the past 3 1/2 years against Ronald Reagan and his friends. Nothing would have pleased me more the past few years if Ronald Reagan had disowned his past anti-worker, anti-union record."

Faced with the beginning of the chorus of charges from the administration, Reagan told reporters traveling with him in Ohio: "I think all of this again is a little nit-picky trying to divert us from the real issues."