Much has been written in recent days about the supposed similarity - not just of views, but of personalities - between President Carter and the man he meets today, Indian Prime Minister Morarji Desai. Publicists would have the world believe that since both men - elected at about the same time and under somewhat similar circumstances - are deeply religious, have a tendency to moralize and share a dislike for excesses of thought or deed, then it follows that one is almost a carbon copy of the other.

Nothing, as Carter will almost certainly find out today, could be farther from the truth.

Desai, a man who at 81 is one of the world's most durable polticians, is as different from Carter as Mahatma Gandhi - whom Desai regards as little short of saintly - was from that most Gandhian of Americans, Martin Luther King. Desai's personal asceticism makes Carter appear a Sybarite by comparison: his regime of exercise and self-restraint makes Carter sound flabbily unfit; his religious views are so strict as to make the U.S. President appear doctrinally lax.

Much is made in America of Carter's early-rising habits. Down at the Indian prime minister's residence in south Delhi there is polite laughter at the news of Carter's 5:30 alarm call. So late! Desia is up and feasting on carrot juice at 4 a.m. each day, is well into his two hours of yoga by the time the White House lights are snapped on, and is halfway through his list of morning appointments by the time most Washingtonians are turning in the first snatches of WTOP news.

The dictary fads of Desai are, by now, too well known to bear detailed recounting. Six ounces of his own urine drunk every morning, a few more ounces rubbed over his skin. An aversion to ice in his drinks, a strict adherence to vegetarianism, a daily intake of less food than the average American would regard as a decent lunchtime side salad. And of course, his total aversion to alcohol even Indians, who like their beer (especially in northern states like the Punjab) find his constant exhortations to his fellow-countrymen to give up alcohol a little tiresome.

But to concentrate on his peculiar personal habits is to obscure the political ambitions that have characterized him since Desai entered government administration in 1918 as deputy collector of the west Indian city of Ahmedabad at the age of 22.

From the start Desai, a skilled administrator and economist, was opposed to British subjugation of his country. As soon as Gandhi began his campaigns to force Britian to "quit India" in the 1930, Desai was by his side.

"I have not changed my political beliefs one whit since I joined Gandhi in 1930," he said recently - and ever since then, both in the struggle for independence achieve in 1947 and in the three decades since, Desai has adhered firmly to the Gandhian principles of self-reliance, non-violence and submission to God.

Desai's progress within the higher echelons of the India he helped build has not been smooth. His first Cabinet post came in 1953, when he was 62 years old: Jawaharlal Nehru, then India's prime minister, offered Desai the job of finance minister - a position which he held on and off, for 11 years.

He resigned once from Nehru's Cabinet, but was recalled by former Prime Minister Indira Gandhi in 1966, when he was given the additional post of deputy prime minister. Three years later, after a dispute over Mrs. Gandhi's economic policies, he quit, and he left the Congress Party he and Mahatma Gandhi had helped form many years earlier.

His enmity toward Mrs. Gandhi - largely a result of his firm belief that she was leading India away from the Gandhian principles he espoused - eventually forced him and that other great-Gandhian Jayaprakash Narayn of the northern Indian state of Bihar, to form an alliance in 1975.

The opposition campaign mounted by Desai and his followers led to the state of emergency proclaimed in July that year - and, inevitably, to the two-year prison sentence that Desai and some of his colleagues had to suffer as a result.

Desai was released almost exactly a year ago to fight and win the most dramatic election that this world's largest democracy has ever staged. It was an election in which Gandhian views triumphed over Mrs. Gandhi's oligarchical style of government, and it appeared to set India on a totally new course.

Desai owns neither land nor house and sits at his spinning wheel for hours on end, turning out homespun cloth in the manner of the great Mahatma.

He is a man who seems, at times, ill at ease with the rush of contemporary life and impatient with the demands of global diplomacy.

The Indian prime minister will seem, to the visiting Americans today, a man of quintessentially Indian mystery.

Without a doubt, he is a man who engenders enormouse respect in those who encounter him. Without a doubt he is a man whose views are so rigid that negotiators and to label him as obstinate, old-fashioned or out of touch. According to those who know him well, however, he is a man of warmth and kindness to those he likes and with whom he enjoys some spiritual communion.

Perhaps the superficial similarity between Desai and his guest today will be enough to insure that by the time this much heralded visit is over, those two newcomers to the political center-stage will part good and lasting friends.