Rebutting the Reagan administration's own report card--and its high marks for deregulation--consumer activist Ralph Nader has sent President Reagan a letter citing 68 administration actions that, Nader charged, would injure the safety, health and buying power of American consumers.

"Throughout our country, helpless people, children, the elderly, the disabled and millions of other innocent consumers are being denied the protection of their national government," Nader said in his Jan. 18 letter.

"From auto safety to antitrust to banking to food safety, your administration has joined with reckless business powers to strip away safeguards vital to consumers. By abandoning sound government regulation of companies you are permitting these companies to privately regulate consumers," the letter said.

In rebuttal, assistant White House press secretary Mark Weinberg issued a statement that said: "The president believes that much federal regulation is wasteful and unnecessary and that the consumer's best protection is not a growing federal bureaucracy but a free and competitive economy.

"This administration will not hestitate to issue and enforce regulations where there is real evidence of need. We are confident our regulatory reform efforts are contributing to a sounder and more competitive economy and to substantial improvements in the welfare of consumers."

To support his claims that the administration's appointees are unleashing "all the worst elements of irresponsible corporate behavior," Nader cited actions in the areas of auto, aviation, food, drug, nursing home and consumer product safety, the easing of antitrust enforcement by the Federal Trade Commission and Justice Department and Federal Communications Commission proposals to eliminate the Fairness Doctrine and the equal-time requirements.

Among the actions that provoked the strongest objections by Nader were:

* Rescission of the requirement for airbags in automobiles by the Transportation Department.

* Cuts in the staff of the Consumer Product Safety Commission by 30 percent.

* Deep cuts in the woman-infant feeding program, which "has been shown to reduce infant mortality."

* Decisions to drop administration opposition to sugar price supports and to leave the tobacco price support program untouched.

* Current and potential future cuts in the school-lunch program and possible elimination of the summer food programs and the special milk program--cuts that the administration says would save $450, or less than 35 percent of the annual cost of sugar price supports.

* Prohibitions against Legal Services Corp. attorneys filing class-action suits on behalf of low-income consumer groups and arguing on behalf of low-income consumers' before legislative bodies.

Nader closed by urging the president, Vice President Bush and consumer adviser Virginia Knauer to meet with a delegation of consumer groups "as soon as possible."