President Reagan's offer to sell 23 million tons of U.S. grain to the Soviet Union in the current crop year was characterized by Soviet and Western observers here today as a ploy to influence the outcome of the Nov. 2 election in the American farm belt.

In an unusual commentary, the government news agency Tass described Reagan's speech yesterday as "clearly designed to make a favorable impression on the farmers on the eve of the Nov. 2 congressional and local elections."

Western diplomats and businessmen who follow the grain trade said there were no indications that the Soviets are prepared to buy more than 15 million tons of U.S. grain in the current crop year, which began Oct. 1.

"And that would be a very good year for us," said one official intimately involved in grain sales.

As if to underscore their reluctance to become overly dependent on the United States and its key allies, the Soviets are reported to have turned down a French offer of guaranteed annual sales of French wheat of up to 3 million tons in the next three years.

Moscow and Paris concluded an agreement yesterday for import of French farm products through 1985 that included corn, sugar, beef, grain and wheat flour. While the details of the agreement were not made public, French sources said the Soviets would not commit themselves to importing more than 1.6 million tons of French wheat annually.

Although it did not mention the details of Reagan's offer, Tass said the president "was forced to admit that the U.S. grain embargo against the Soviet Union imposed by the Carter administration in 1980 had been a total failure." It added that Reagan "promised the farmers his administration would not repeat that mistake."

The commentary also struck the themes that have been sounded publicly and privately -- that the United States is an "unreliable" and capricious trading partner, that various U.S. embargoes had little or no effect on Moscow since it can get supplies elsewhere, and that the Americans were shooting themselves in the foot by relinquishing lucrative trade deals to others.

Citing American figures on farmers' indebtedness, bankruptcies and unemployment among agricultural workers, Tass said that "the plight of the agricultural areas in the United States has drastically worsened the position of the Republican Party in those districts and the president is now seeking to redress the situation at all cost."

Diplomatic analysts said the Tass dispatch suggested that the Soviets intend to continue to buy American grain. At the same time, it contained a hint that grain trade is one place where Moscow has some leverage to stir political trouble for Reagan in the farm belt of the Midwest.

From an economic standpoint, Moscow is still forced to maintain access to the U.S. farm market. But there is no doubt, according to experts here, that the Carter embargo, followed by Reagan's "trade war" against Moscow, has had a lasting effect on Soviet buying patterns.

While American farmers held nearly 75 percent of the Soviet food import market in the year before the Carter embargo, that share has dropped to slightly above 30 percent in the crop year that ended Sept. 30. This is despite the fact that Moscow bought 14 million tons of grain during the past year, the second highest amount ever.

After three consecutive disastrous harvests, Soviet import needs have grown. Moreover, the Soviets are facing a very poor harvest for the fourth year in a row.

Western experts here say that Argentina has received assurances that the Soviet Union will buy 10 million tons annually. Canada and the Soviet Union have made a deal for the sale of 8.3 million tons of Canadian grain, and the quantity may go up. Moscow can buy between 6 million and 8 million tons from Western Europe, Brazil and several other countries. The remaining amount to meet Soviet needs of between 40 million and 46 million tons is expected to be bought in the United States. This amount, acording to these experts, is in the range of 10 million to 15 million tons.