President Jose Napoleon Duarte has lost much popular support in the past year because of his conservative policies and a series of political mistakes, according to numerous observers here, including members of his Christian Democratic Party.

Organized labor, once a major source of backing for Duarte, has swung against him. Union leaders charge that the president has reneged on his campaign promise to seek a negotiated settlement of the nation's civil war and has adopted probusiness economic policies.

Duarte also has disappointed many nonunion Christian Democratic backers, who say that he increasingly is being viewed as a pawn of the United States. They charge that he has backed off his own populist positions in domestic affairs and has fashioned a foreign policy in Washington's interests rather than his own.

On another political front, military officers still are grumbling over Duarte's decision to negotiate with left-wing guerrillas to obtain release of his kidnaped daughter in October.

Military sources said that the armed forces does not want to stage a coup, which almost certainly would result in a cutoff of U.S. military aid, but that the officer corps is more disillusioned with Duarte than at any time in his nearly two years in office.

In one sign of the times, bumper stickers have appeared in San Salvador saying, "I didn't vote for him." Readers know who "him" is.

Duarte and other government spokesmen respond to the criticism by insisting that the administration has done the best it could under difficult circumstances.

The political climate contrasts sharply with that of a year ago, when the Christian Democrats won control of the Legislative Assembly in a surprise electoral victory. The feeling then that the country was moving forward has evaporated, and a mood of resignation and stagnation now prevails, political observers said.

Duarte has made little progress toward solving the economic and social problems that contributed to the rise of the leftist insurgency in the late 1970s and early 1980s. Government spending on health, education and other social programs is lower now than in 1978, after discounting the effect of inflation, even though U.S. aid has ballooned from less than $10 million annually to more than $300 million.

Critics acknowledge that Duarte has presided over the consolidation of gains achieved in late 1983 and 1984 in curbing right-wing political violence and battling the Marxist-led guerrillas. He still benefits politically from those improvements, although they had their roots in U.S.-backed changes in the armed forces that occurred six months before he took office.

Duarte still possesses considerable political skills and also benefits from deep divisions among his opponents on the political right.

In addition, Duarte's mere survival as a civilian president is seen as an advance for democracy in a country where the military has ruled most of the time.

It is a measure of the changes in recent years that observers today worry about relatively conventional political issues, such as Duarte's troubles with the unions and the abilities of his Cabinet. As recently as the end of 1983, it looked as though the leftist rebels might seize power or that right-wing death squads would prevent a centrist such as Duarte from taking office.

Nevertheless, Duarte is struggling to regain the political initiative. His biggest challenge is to win back support from the labor movement, which has become the focus of nonviolent opposition to the government in recent months.

"What worries me most is what is happening in the labor sector. This is going to become a factor of great influence," a high-ranking Salvadoran official said. He and several other observers said that Duarte had made a major political blunder in neglecting to maintain close communication with union leaders.

Several unions and union factions that once supported the Christian Democrats have now severed ties with the party. These groups, which represent at least 50,000 workers, have joined a new labor confederation that includes Marxist-oriented unions with links to the guerrillas.

"The government did not fulfill the promises of the social pact with the unions in any way. It has lost most of its social base," said Miguel Angel Vasquez, president of a public workers' union that deserted Duarte for the new confederation.

The new labor confederation, called the National Union of Salvadoran Workers, held a march on Feb. 21 to protest Duarte's economic policies. It drew only about 15,000 participants, but that was the largest demonstration in El Salvador since 1980.

The Christian Democrats managed to outdo the unions by organizing a pro-Duarte demonstration this month that drew about 20,000. But the government had to bus in large numbers of marchers, and the effort reflected the degree of its concern.

The unions have been particularly critical of Duarte's economic package, which was unveiled in late January. They object to a 20 percent increase in bus fares, and hikes in fuel prices that also were expected to spur inflation by adding to transportation costs.

The confederation's economists estimated that the cost of producing corn, the nation's basic foodstuff, had risen 62 percent under the governmental package. The price paid by the government for corn had risen only 43 percent, however.

The package included several measures aimed at the upper classes, such as new tariffs or outright bans on imports of more than 200 luxury items, including champagne and large automobiles. But businessmen, union leaders, diplomats and Christian Democratic politicians consulted were almost unanimous that the burden of the economic package fell largely on the middle and poorer classes.

"It's a totally capitalist package, in the style of those recommended by the International Monetary Fund," said a senior Christian Democrat. Asked why Duarte had accepted such measures, given his party's historically populist outlook, the source shrugged and answered: "U.S. influence. There is no alternative. Who is giving us the dollars?"

Union leaders, businessmen and U.S. officials also accused the American Institute for Free Labor Development of weakening Duarte's union base. The institute, which is funded jointly by the AFL-CIO and the U.S. government, used its financial and political influence in late 1984 and early 1985 to try to push several unions to adopt more conservative political positions.

Several unions, and union factions, balked at the pressure and ended up joining the new, more radical confederation. A U.S. official said that the institute's strategy "totally backfired."

Duarte's economic package won fewer points with the business community than might have been expected. The nation's wealthy always have distrusted him. When Duarte claimed prematurely that businessmen and landowners supported his measures. When private-sector spokesmen disputed him, Duarte attacked the well-to-do, saying they were trying to sabotage him.

Duarte's foot-dragging on peace talks with the guerrillas has disappointed unions and Christian Democratic factions. He has shown little interest in promoting a dialogue with the left, one of his main campaign pledges, since the last talks with the rebels in November 1984.

To a large extent, Duarte is caught in the middle on the issue of peace talks. The armed forces insist that he operate within the limits of the 1983 Constitution, which means that the guerrillas could win a political role only after ending their insurrection and running for office.

The rebels say the Constitution is irrelevant and that their military strength and popular support should give them a share of power without competing in elections.

Duarte apparently hurt his own cause on March 4 by saying he would meet again with the Salvadoran guerrillas if Nicaragua's President Daniel Ortega would meet with the Nicaraguan rebels.

The surprise proposal had every hallmark of being designed to please Washington, which has been pushing Ortega to meet with the U.S.-backed counterrevolutionaries. El Salvador's Roman Catholic Archbishop Arturo Rivera y Damas criticized Duarte's as insincere.