The early promise of President Miguel de la Madrid's administration has faded considerably as he reaches the midway point of his six-year term on Sunday, Mexican and foreign political observers say.

The president's economic management, which was awarded high marks in the early days of his term, increasingly is being questioned. The administration moved too early to pump up the economy last year, and now it is paying the price by having to return to austerity at a time when its original strategy called for a healthy economic upturn.

In addition, the president's much-publicized anticorruption campaign has languished. Even the government's main international initiative, the Contadora peace effort in Central America, appears to have reached a dead end.

Criticism is mounting that de la Madrid's low-key, technocratic style of leadership has left the country without a clear sense of direction. Mexican political scientists, foreign diplomats and even some government officials said in interviews that the public had lost confidence in the government, which frequently has seemed indecisive or hesitant to exert its authority.

"This administration has no political credit left. It couldn't afford to make mistakes, but it did," a government official said privately.

The "sense of drift," as a Western European diplomat called it, is particularly worrisome because it has surfaced so early in the president's term.

Historically, Mexican presidents' programs often have swerved off course in the closing months of their terms because they were losing influence or were trying to win a place in history through dramatic populist measures. But several analysts said that de la Madrid was acting like a lame duck before his term was half finished.

The president's defenders stress, and his critics admit, that de la Madrid inherited a very bad situation from his predecessor, Jose Lopez Portillo.

When de la Madrid was inaugurated on Dec. 1, 1982, the nation already was in the grips of its worst economic crisis in decades. The outgoing administration was considered extremely dishonest, as Lopez Portillo reportedly enriched himself during his term by hundreds of millions, and possibly billions, of dollars.

"The administration's principal achievement is to have survived. It has maintained political stability in a situation without precedent in modern Mexico," Lorenzo Meyer, a political scientist at the private College of Mexico, said.

In addition, a variety of observers agreed that disappointment with the government has not reached the point where there is a serious risk of violent change in the social and political structure.

Most Mexicans accept that there is no alternative to de la Madrid's Institutional Revolutionary Party, known by its Spanish initials PRI, which has ruled Mexico for more than 50 years, the analysts said. The party's method of government -- a mix of bureaucratic patronage, ideological pragmatism and vote fraud -- is reminiscent of that used by former Chicago mayor Richard M. Daley in his heyday.

"The PRI, with all its warts, is still a known quantity," said Cathryn Thorup, a specialist in Mexico at the Overseas Development Council in Washington. She added: "I've been following Mexico for 10 or 12 years, and people always say that it can't go on like this, that it has to fall apart in five years or so. But that date always gets pushed back."

The economy is the president's biggest headache, and it remains a question mark despite stepped-up government efforts beginning in the summer to put things in order.

The government is widely criticized for focusing too much on technical economic solutions, and for neglecting to do the grass-roots political work necessary to preserve popular support. As a result, the government has managed to irritate both unions, which are upset over lagging prospects for economic growth, and businessmen, who want sharper cuts in government spending.

"This administration is very technocratic, and this is serious. Mexico has always been led by very able politicians," said Rafael Segovia, a generally progovernment political scientist.

In 1983 and early 1984, the International Monetary Fund and foreign bankers held up Mexico as the example for other Latin American debtors to follow. Under de la Madrid, the country brought down its inflation rate from 100 percent in 1982 to 60 percent last year, and it was the first nation to negotiate postponements in repaying the principal of its $96 billion debt.

Around the middle of last year, however, the Mexican government went on a public spending binge, apparently to get the economy moving before last July's legislative and state elections. As a result, the country has fallen well short of the targets set with the IMF for cutting inflation further and for narrowing the budget deficit. Inflation this year is expected to remain at about 60 percent, or far above the target of 35 percent.

Now Mexico is seeking to negotiate a new IMF agreement, which it needs to convince foreign bankers to make fresh loans in 1986. But it had to promise to return to the kind of austerity measures, such as tight monetary policies, reduced subsidies and higher taxes, that contributed to a sharp recession in 1983.

The government predicts that there will be virtually no economic growth next year. De la Madrid acknowledged in his annual budget message two weeks ago that his administration's original plan of achieving at least 5 percent annual economic growth from 1985 until 1988 "does not appear viable."

Many of Mexico's economic problems are the result of international factors beyond the government's control. The price of the country's main export, petroleum, has sagged on world markets.

But many sources said that the government's indecisiveness had caused much of the damage this year. The government lost a billion dollars or more in oil sales in the spring and summer when it dragged its feet about lowering its prices to compete with members of the Organization of Petroleum Exporting Countries who were ignoring that group's pricing guidelines.

The administration also broke one of its own cardinal rules at about the same time by waiting too long to devalue the peso, leading speculators to transfer large sums out of the country to invest in dollars.

"When de la Madrid looks at his options and none is good, then he won't do anything. But you don't govern Mexico just with figures. You have to create confidence," a senior diplomat, who asked to remain anonymous, said.

The president has regained some initiative in recent weeks with his pledge to slash the budget deficit in 1986 and his decision to lower trade barriers by joining the General Agreement on Tariffs and Trade. The peso, which had fallen to a low of 500 to the dollar in the uncontrolled exchange markets, has recovered slightly. Prices also have soared on the small stock market.

But businessmen and diplomats expressed concern that the government again would fail to meet its own economic targets, and they warned that time was running out. A progovernment business leader, Ernesto Amtmann, said the administration has only 18 months left to make major initiatives before it is swamped by such problems as infighting among government ministers jockeying to become the next president.

"We can't spend any more time thinking about what to do. We have to act," Amtmann, who is chairman of a diversified chemicals and pharmaceuticals company, said.

In other fields, the president's call for "moral renovation" has had little impact. Several prominent officials from the last administration have been arrested on corruption charges, but none has come to trial.

Several sources said that government officials had to be more cautious now about demanding kickbacks or bribes, but they agreed that payoffs continue. A businessman, who asked to remain anonymous, said he was asked to pay a kickback of 10 percent of the value of his contract to obtain a permit to import powdered milk.

"There are so many people involved in this, and it has been going on for so many years, that it's impossible to get rid of it altogether," a government official said. He criticized the U.S. media for having unrealistic expectations: "The problem with you guys is that you want an overnight change, and that can't happen."

One of the worst blows to the president's "good government" campaign was the open vote fraud in gubernatorial elections in the northern states of Sonora and Nuevo Leon in July. Many observers said that the president genuinely had wanted clean elections, but lower ranking officials ignored him.

The president's apparent lack of influence over his own government was mocked in a pair of riddles heard in government circles: "Why is de la Madrid like Tarzan? Because both are surrounded by animals. Why is de la Madrid unlike Tarzan? Because the animals listen to Tarzan."

Finally, in the field of foreign policy, Mexico's backing for the Contadora peace negotiations in Central America has alienated the Reagan administration while achieving nothing except a series of stalemates in bargaining. This is hardly de la Madrid's fault, as differences currently seem to be insuperable between Nicaragua and its more conservative neighbors Honduras, Costa Rica and El Salvador.

Nevertheless, the administration invested a lot of prestige in the Contadora process and has little to show for it. Government officials now speak privately of Contadora as a well-intentioned but utopian effort.