President Carter yesterday personally announced the approval of $670 million in government credit guarantees to Poland that will allow that Eastern European country to purchase about 4 million metric tons of grain and other agricultural products from American farmers.

The amount of grain to be purchased with the credit guarantees during the fiscal year that begins Oct. 1 is not significantly larger than the amount purchased by Poland this fiscal year.

But circumstances have changed in Poland and the United States, apparently prompting the unusual appearance before television cameras by the president for an announcement normally made by the Agriculture Department.

Poland has just come through a period of intense labor unrest during which the Carter administration sought to encourage the movement toward independent labor unions without provoking intervention by the Soviet Union. Yesterday's announcement was clearly meant as a signal, in Carter's words, of the continued "solidarity between the American people and the Polish people."

What has changed in the United States since last year is that the president is running for reelection and is thus particularly sensitive to such key voting blocs as farmers and Polish Americans.

There is still unrest in farm states over the embargo of grain shipments to the Soviet Union. Yesterday's announcement may serve to placate that somewhat. In addition, the administration has been under pressure from Polish-American groups to make some concrete sign of support for Poland in the wake of he labor unrest.

Carter's announcement of the credit guarantees to Poland was only part of a day devoted to the pursuit of ethnic votes.

Earlier, the president personally awarded the Medal of Honor to Anthony Casamento, an Italian-American World War II veteran who spent years lobbying the government to award him the medal for his actions as a Marine corporal on Guadalcanal.

Carter's pursuit of the ethnic vote will continue tonight, when he is to follow Republican presidential nominee Ronald Reagan in addressing the Italian-American Foundation dinner in Washington, and next Saturday, when he is to fly to Chicago to speak at a Polish-American function.

The Medal of Honor ceremony was attended by a number of Italian Americans, including two members of Congress who used the occasion to attack Reagan for his use of the term "Mafia."

Rep. Mario Biaggi (D-N.Y.), and Rep. Geraldine A. Ferraro (D-N.Y.), the deputy chief of the Carter reelection committee, called on Reagan to clarify his use of the term they said is "an offensive one which Italian Americans here worked hard to erase from common usage."

In an interview pubished in The New York Daily News, Reagan was quoted as saying that as president he would not surround himself with "any little local Mafia." This was a reference to Carter's senior aides from Georgia, sometimes referred to as "the Georgia Mafia."

Biaggi and Ferraro denied that Carter aides orchestrated their denunciation of Reagan, which took place on the driveway of the White House. d

White House officials said the $670 million in credit guarantees for Poland is the largest amount ever offered to a single country and will account for more than one-fourth of the total of $2 billion in credit guarantees to be provided to all countries for agricultural purchases next fiscal year.

Poland received $550 million in credit guarantees during the current fiscal year for the purchase of 3.8 million metric tons of grain and other agricultural products. A metric ton is about 2,200 pounds.

White House press secretary Jody Powell, seeking to justify the president's decision to make the announcement himself, said this was the first time the United States had provided Poland with all it had requested in terms of agricultural credit guarantees.

Powell said the Polish government has pledged that none of the grain will be transferred to the Soviet Union, thereby undercutting the Soviet grain embargo, and he noted that"the Polish people would be upset to learn of it" if such a transfer took place.

Carter told reporters that the United States was "acting on an urgent basis" in responding to the Polish request. He also said the recent unrest in Poland was another sign that "the yearning for human rights is one of the most powerful and constructive" in the world.