The night before Mexico's President Jose Lopez Fortillo nationalized the banks, he called top members of his Cabinet to his official residence.

He had made the final decision without consulting his treasury minister, his foreign minister or his interior minister, according to informed government sources. What he wanted at the meeting was to know whether they were with him or against him, and one by one he had them pledge their fealty.

The next morning, by one published account, when the director of the Foreign Trade Bank began to criticize the measure at an official breakfast, Lopez Portillo cut him off. "You take it or leave it," said the president.

This image of Lopez Portillo as a man alone deciding almost defiantly to take the most drastic measures of his term three months before its end has become central to the immediate political success of the measure. But in the opinion of businessmen and some financial analysts, it may also be the root of further economic disaster.

On the one hand, the boldness of the Sept. 1 nationalization of all domestic banks and the fact that the people most directly and obviously affected are an upper class often seen as abusing its privileges, made the president's decision very popular.

On the other hand, the apparent capriciousness of the move has shaken the much needed confidence of many investors. In a matter of minutes the rules of the game changed radically for business men and anyone else concerned with the country's financial life, and the implicit threat remains that further drastic measures could be taken even before Lopez Portillo leaves office on Dec. 1.

Meanwhile, government policy makers, including supporters of the nationalization, have scrambled frantically to create ad hoc regulations they must constantly clarify and reclarify.

In the confusion, the possibility has emerged that private enterprise on a large scale could become a vestige of the past.

"That is the fear we really have," a prominent business leader said this week.

Many businessmen hope that Miguel de La Madrid, who becomes president in December, will somehow reverse Lopez Portillo's actions.

Some sources maintain that the Harvard-educated former planning and budget secretary was himself left in the dark about the decision to nationalize until the night before it was announced, and the apparent listlessness of his applause as seen on television during Lopez Portillo's public announce ment of the measures fueled speculation that he opposed it.

But, in accordance with Mexican political decorum, de la Madrid has limited him self to broad generalities concerning the measure and has made no statement that could be construed as opposition.

In a speech Thursday, de La Madrid simply reaffirmed his goal of economic developemnt in the context of "political and social pluralism and the mixed economy,"

"The country is undergoing especially difficult moments. President Lopez Portillo firmly holds the helm in the middle of the storm," de la Madrid continued. "I reiterate today my political solidarity and my personal affection for him,"

Meanwhile, the threat of rapidly expanding statism is considered so great by private-sector leaders that they have called hurriedly for a mass meeting of businessmen from all parts of the country this month to decide what they might do.

The government acquired interests in hundreds of major businesses ranging from department stores to restaurants, hotels, insurance companies and tractor factories when the banks that previously held them were nationalized.

Government spokesmen say they have "no intention" of keeping these interests and will sell them or devise some way to use them as partial compensation to the former owners of the nationalized banks.

But the powerful labor movement, which is intimately tied to the government party and shows increasing influence upon Lopez Portillo, has come out against such a move.

Even if the government does finally put the stocks of such companies on the Mexico City exchange, businessmen and diplomatic analysts question who will have the money or desire to buy them, given the current environment.

Yesterday business leaders visited with Lopez Portillo to attempt once again to smooth over differences. The president reportedly reiterated and emphasized his constitutional authority to nationalize the banks, while the businessmen called for an end to confrontation and even contributed several hundred thousand dollars to a government-sponsored fund to pay former owners of nationalized banks.

But despite such protestations of good will, an atmosphere of bitter confrontation between business and government is growing steadily. The "productive alliance" between the state and private business interests with which the president began his term six years ago has been severely damaged, and some analysts say it has been broken altogether.

Businessmen accustomed to influence and power now find themselves baited and vilitied as the government organizes mass demonstrations of support for the nationalizations.

Some factory owners threatened a Lockout this week to protest the bank takeovers, then hastily called it off, at least partly for fear that it would give the president a pretext for further nationalizations.

Manuel J. Clouthier, director of the Business Coordinating Council that represents virtually all major elements of private enterprise in Mexico, was reluctant to talk about such concerns in an interview this week, saying only, "I hope there won't be anything more" in the way of nationalizations.

"We want to be very prudent in our actions," said Clouthier. "We do not want to do anything that could put us outside the law," as some officials suggested a lockout would do.

"We have all paid something in this," said Clouthier. "I think that labor has done its share. I think the entrepreneurial sector, which is really ruined now because it already was almost bankrupt, has paid, and then so me. I have the enormous desire to see public officials begin to do their share, too."

In fact, although many public officials are noted for the enormous wealth they accumulate in office, popular resentment remains focused on businessmen, who often seem to use government protection to turn out inferior products, or to gouge customers.

Bankers were particularly vulnerable to charges of abuse. As one American financial analyst said, "Banks here were very, very profitable. A bank here might make as much money as one that was two or three times as big in the United States. "

Carlos Tello, the new director of the Bank of Mexico and one of the principal authors of the nationalization, said in a recent speech that the banks were charging 40 percent more for the money they loaned than they paid to borrow it. Other government officials have said that bank profits went up 300 percent between January and April, even as the country as a whole was sliding into the worst economic crisis since the Mexican Revolution in the early part of this century.