Responding to charges by Ronald Reagan that military weakness has contributed to U.S. problems in the Persian Gulf, President Carter claimed yesterday that the United States has "clear naval and air superiority in the whole region" and accused Reagan of making false claims that diminish allied confidence in this country.

Candidates in every election charge that the Soviets lead the United States in military strength, the president told a town meeting in Nashville, Tenn. After the election, the charge is proven false, he said.

Carter told his audience that political candidates "make a mistake when they run down America's strength, when they say falsely that America is weak."

The president's attack yesterday on Reagan was part of a double-barreled counteroffensive launched jointly with Secretary of Defense Harold Brown against critics who have charged that U.S. combat readiness has been allowed to slip dangerously in recent years.

Addressing a Chamber of Commerce and Rotary Club audience in El Paso, Tex., Brown claimed that recently published reports indicating lack of readiness of many Army divisions based in this country, Navy ships and Air Force squadrons were "extremely misleading" because they reflect peacetime measurement techniques and are not an indication of what the United States could field in a wartime emergency.

"Our forces are ready to go to war -- if need be -- and we are increasingly able to sustain our forces in combat," Brown said in what the Pentagon billed as a major speech.

If your car is sitting in your driveway and you are not supposed to drive it because the backup light is out, he told the audience in an effort to illustrate the difference between peacetime and wartime standards, it does not mean you would not use it to drive your child to the hospital in a middle-of-the-night emergency.

Brown called for "perspective" on the defense issue, reminding his audience that for many years the United States essentially mortgaged future spending on more modern weapons to pay for the cost of the Vietnam war and that when the United States came out of that war in 1973 it faced a huge rebuilding job. He said there have always been periods in history when this nation has had to focus on buying new arms and other periods when it has had to concentrate on making existing forces as ready as possible to fight.

The rebuilding job has been going on for several years, he indicated, and now the Pentagon is able to give the readiness issue higher priority.

Brown pulled a leaf from Republican challenger Reagan's book to make his point. Reagan has frequently quoted Democratic President Franklin D. Roosevelt. Yesterday, Brown pulled out Republican President Eisenhower's warning that "there is no such thing as maximum military security short of total mobilization . . . with all the grim paraphernalia of the garrison state."

In claiming that "we are stronger today than we were five years ago, and we will be stronger five years from now than we are today," Brown made the following main points:

Though the Army has decreased in size, Brown claims that since 1973 it has gained significantly in firepower, while restructuring has increased the divisions from 13 to 16 and added eight more combat battalions.

Air Force tactical fighter and attack plane strength has grown from 32 to 35 wings since 1976, with an additional 100 such planes added as of this year and about 400 more scheduled for the next few years.

The U.S. fleet, which Brown said was allowed to shrink drastically from its Vietnam peak of 1,055 ships in 1968 to 531 ships in 1978, is "now on the road back, and the Navy is growing again." This year there are 540 ships and another 50 are expected to be added by 1984, he said.

To buttress his claim that "we have not been standing still," Brown said the United States had shipped 51,000 new antitank missiles to divisions in Europe in the past four years. Ammunition storage in Europe had been increased by 55 percent since 1976, he said. And, indirectly acknowledging a problem, he added that by the end of fiscal 1981 "we will have prepositioned adequate stockpiles of war reserves to support our forces in the early stages of a NATO conflict."

Giving credit to previous admintrations for key investment decisions made in the aftermath of Vietnam, Brown also reminded his listeners that the new submarine-launched Trident missiles are now entering the fleet, and said that U.S. F14, F15 and F16 fighters are now the best in the world and that the new A10 attack plane is the best "tank-killing" aircraft flying today.

He also pointed to this administration's support of the new air-launched cruise missile program, the beginning of mass production of a new tank for the Army and the advanced attack helicopters that, beginning next year, will augment the Army's existing fleet of 1,000 attack helicopters.

In judging overall readiness, Brown said it must be kept in mind that many Soviet divisions are at lower levels of readiness than U.S. units, that frontline American divisions overseas are in good shape, that U.S. allies are more reliable than Soviet allies and that the essentially defensive posture of the western alliance did not require as much weaponry as aggressors require.

Brown said the core of the readiness problem is the shortage of skilled enlisted people, many of whom are leaving the services for better paying jobs outside. He said this is not a new problem, that the Navy's shortage of about 20,000 petty officers is about the same today as it was in 1976. Brown said the newly approved pay raise would help but he declined to explain why the administration took so long in trying to improve military pay and why, until recently, it opposed a sizable increase.