Even for Mexico, where corruption is considered as common as the daily diet of corn, it has been a dizzying month.

First the governor of Coahuila, a state bordering Texas, was forced to resign after "inexplicably" making $30 million while in office. Then the two top officials of Mexico City's airport were formally charged with large-scale fraud and dealing in contraband. A congressman for the government party was investigated for embezzlement of nearly $80 million in state funds. And finally President Jose Lopez Portillo himself, pained and embarrassed, turned down the gift of a $2.4 million ranch after a newspaper disclosed the donation.

In the wake of all this, the readers of Mexico City's 21 newspapers are being treated to many new versions of the old debate over the arrogance and relative impunity of public officeholders, and whether they may be limited at last.

Mexicans have seen spectacular corruption trials before, and the common sight of officials emerging with a fortune after their six-year terms has made people unswervingly cynical. Ask a Mexican about the honesty of public servants and he will say something rude. There is even a popular verse to describe the last year of office: "At the end, only a fool leaves anything behind."

Public opinion is therefore not easily swayed when it comes to anti-corruption campaigns.

If Mexicans are not surpised when an official is caught with his hand in the till, they still love to read the press accounts to savor the inner workings of a fraud.

Reading about the Coahuila governor the other day, a white-collar aficionado of political scandal said: "This administration has arrested more officials than any other. To us Mexicans this confirms two things: one, that just about everyone who gets a chance will do something crooked, and two, the fortunes to be made in public service are even bigger than we thought."

The story of Oscar Flores Tapia, the governor of Coahuila who fell from grace in mid-August, is as much a tale of the impressive upward mobility of Mexican society as it is of alleged large-scale fraud. And politics is one of the best ladders.

The governor's campaign pamphlets said he grew up as a very poor boy in a neighborhood where the "homes were inhabited by misery."

He stepped into public service as a traffic policeman, all the while developing his business talents. He was already prosperous by the time he became state governor five years ago. Salaries of public officials are kept secret in Mexico, but now a congressional grand jury has charged him with violating a new law called "inexplicable enrichment." Over the last six months, the charges state, the governor and his family acquired properties worth $30 million, which is "well beyond their economic possibilities."

"Whatever earnings the ex-governor cannot explain as honest business, he will have to give back to the government," said Attorney General Oscar Flores Sanchez.

"The governor had a reputation for being nutty," said Flores Sanchez, "but how could the president know he would turn out like this? Only the watermelon is cut open before you buy it."

The case of the president's ranch, dubbed the "ranch of temptation" by a local magazine, has made even more waves. Never before has a Mexican president been challenged seriously on such delicate personal grounds while still in office. Both the person and the office of the presidency have been off-limits here in an unwritten but almost sacred law. Ferocious criticism starts as the man steps down.

It all began early this year, when a group of businessmen and politicians from Mexico state bought a grand old ranch house and remodeled it with the president's hobbies and tastes in mind.

A shooting range, swimming pool, tennis and archery courts, a gymnasium and artist's studio were added specifically for Lopez Portillo, who likes sports as well as writing and painting. The refurbished home and its 150 acres of grounds, about 50 miles from Mexico City, cost $2.4 million. Lopez Portillo had already taken relatives to see it and, like him, "they were enchanted by the place," he said. The gift would become the retreat of Lopez Portillo, who steps down in December 1982.

Yet it might have gone as unreported as the villas, farms, cars and planes given to past Mexican presidents. The exchange of gifts is, after all, as much a part of politics as of Mexican culture, and the man at the top, like an Aztec king, receives the most lavish ones. But Lopez Portillo had done two things: he had encouraged criticism, even of his own office, and he had made the fight against corruption one of his battle cries.

"Corruption is the cancer of this country," he once said in a speech. Although he has been criticized harshly for giving top government jobs to his close relatives, he has gained a reputation for honesty.

Two weeks ago, a prestigious columnist, Miguel Angel Granados, decided to take the president at his word. He wrote of the impending gift in the daily newspaper Unomasuno, bluntly questioning the principles involved.

The president's answer was quick. He sent Granados a handwritten letter that was published on the front page of Unomasuno the next day. "Your article," he wrote, "as in a mirror, made me see my image reflected in the opinion of my fellow countrymen. The ranch is an ideal retreat for a man without a future in Mexico: a former president." He concluded the long letter by saying, "Although the temptation is great, with true sadness, I have decided not to accept the gift." Political Mexico was stunned.

A few days later, before congress, Lopez Portillo suddenly digressed from the text of his prepared state-of-the-union address and proposed that congress adopt a law to control gifts to public officials, "particularly the president of the republic." He got wild applause.

The politologos, as students of politics here are called, think that with "the ranch of temptation" history has been made. Never mind the public officials grabbed for embezzlement. The milestone laid is the precedent of holding the president directly accountable for his deeds, along with Lopez Portillo's forthright response. From the president's point of view, the politologos say, it was more than an honest move. As a result of it, they believe, he will suffer from less "political cannibalism," the ancient Mexican ritual of tearing apart politicians once they step down from their dizzying heights.

Meanwhile, Flores Sanchez, the tough, outspoken attorney general, is pressing for further legislation against conflict of interest and influence peddling. His office has sent 2,360 federal officials and employes to jail in the last five years and recovered close to $200 million worth of state funds.

"I don't know if corruption is less in Mexico these days" said Flores Sanchez, "but at least people are being more careful. They're not leaving as many fingerprints."

The politologos believe that as the country grows richer and the public coffers swell, the people's faith in the system is reaching an all-time low.

"The anticorruption campaign is obviously vital for the survival of the system," said a prominent lawyer recently. "But the real question is not how much purging does the apparatus need to recover people's faith. The crux of the matter is, how much purging can the Mexican political system take before it starts coming apart at the seams?"