Facing intensifying attacks for its environmental record as the Nov. 2 election day looms, the Reagan administration yesterday attempted to seize the offensive at a press conference called to unveil a ban on almost all major uses of the controversial pesticide, toxaphene.

Once the most widely used pesticide spray in the country, particularly in cotton and soybean fields in the South, toxaphene is suspected of causing cancer in humans and has poisoned large numbers of fish and wildlife. The government has been considering a toxaphene ban since 1977; Congress last month called for sharp restrictions on its use.

Yesterday, Assistant Environmental Protection Agency Administrator John Todhunter announced that the agency will outlaw the widespread spraying of toxaphene on cotton fields and enough other applications to account for 95 percent of the toxaphene used last year. The ban will take effect 30 days after manufacturers are notified.

Todhunter also pointed out toxaphene use has fallen sharply, from more than 100 million pounds a year in the early 1970s to about 16 million today as insects developed an immunity to it. EPA will continue to allow "minor and emergency" uses, which accounted for about 900,000 pounds of toxaphene last year, he said.

But before getting down to specifics, Todhunter took a swipe at his, Democratic predecessors, pointing out that when President Reagan took office EPA had worked on the issue for three years without taking official action. "In contrast to our predecessors, we don't sit around and talk about our commitment to the health and welfare of the country," Todhunter said before nine television cameras. "We're much too busy doing our job for the taxpayers of this country."

Saying that the Reagan administration had inherited "enormous backlogs" of EPA work, including the toxaphene decision. Todhunter declared, "This administration is committed to the conservation of this nation's natural resources."

He added it is "political hogwash" to connect yesterday's ruling to the campaign season. In fact, Todhunter contended that EPA would have acted a month ago had Congress not confused the issue by inserting a toxaphene ban in a funding resolution last month.

Rep. Sidney Yates (D-Ill.), chief congressional proponent of the ban, disputed this claim, calling it "typical of the Reagan administration policy of blaming everyone except themselves. If they can't blame Jimmy Carter, they look for someone else. And here I was."

Despite Todhunter's disclaimers, environmentalists called his comments "an election speech," contending the administration has relaxed rather than toughened pesticide controls.

As Todhunter spoke, four environmental groups filed suit in U.S. District Court here, attempting to reverse EPA's recent decision authorizing "emergency" use of the banned pesticide, ferriamicide, on fire ants in Texas, Mississippi and Arkansas. Ferriamicide contains mirex, which was banned by EPA in 1978 as a health hazard to humans and animals.

The lawsuit contends that there is no "emergency" to warrant use of the pesticide, and that 17 safer alternatives are available.

Environmentalists also have criticized the agency for authorizing use of the banned pesticide DBCP, on peach orchards in South Carolina, following pleas from farmers who complained that they were suffering from an "economic emergency." A court order has temporarily halted DBCP use there.

Under yesterday's ruling, EPA could also authorize emergency use of toxaphene to combat an outbreak of insects resistant to other pesticides, Todhunter said. Citing this provision, a spokesman for a chemical industry group called EPA's toxaphene ruling "reasonable, fair and quite justified."

The chemical's main manufacturer, Boots Fison Chemical, Inc. of Wilmington, Del., also responded calmly to the order. Because of the reduction in toxaphene use, he said, "it's not a major contributor to the company."

Yesterday's ruling followed five years of review under an EPA process that weighs the risks of toxic chemicals against their benefits. In that time, new studies confirmed earlier work linking the pesticide to cancer in laboratory animals and to fish and wildlife damage near fields where it was sprayed.

Recently, the chemical was discovered in fish in the Great Lakes, indicating that it travels long distances through the atmosphere without decomposing, and that it can easily enter the food chain.

Todhunter stressed that the agency's decision was based mainly on toxaphene's threat to wildlife and the environment, since studies have not conclusively linked it to cancer in humans.

The ruling bans the most common uses of toxaphene, including its spraying on cotton and soybean crops to kill pests and weeds. Last year, farmers sprayed 2.6 million pounds of toxaphene on cotton fields alone, EPA officials said.

The ruling allows continued use of the chemical as a dip to prevent skin disease in cows and sheep. EPA staff scientists had recommended in a draft of yesterday's decision that the agency phase out the cattle dip in five years, citing possible health threats to farm workers and to cows.

Todhunter said agency officials determined that the benefits of the cattle dip outweighed the risks, and that dipping could be banned if substitutes become available. Those who apply the dip must wear protective clothing, Todhunter said.