After 18 turbulent months as president of a new democracy, Raul Alfonsin has launched broad economic and political changes that are as perilous as they are dramatic.

The adjustment has been dictated largely by economic conditions, and the response by the 58-year-old leader has been forceful and pragmatic. Nevertheless, the shift has meant new sacrifices for Argentina's crisis-weary population and invited a major confrontation with opponents ranging from powerful unions to sectors of Alfonsin's own Radical party.

As a result, many politicians and diplomats here say whether the president will be able to stabilize his administration, and preserve Argentina's political renaissance, remains an open question.

"It is one thing for the president to change his mind," a diplomat here said. "It's another thing to actually implement a change of course and get the bureaucracy and the party lined up behind it. And Alfonsin hasn't yet shown he can do that."

The major stroke of the president's new strategy came with the announcement recently of a new shock treatment for Argentina's soaring inflation. By freezing wages and prices, introducing a new currency and ordering an abrupt end to the printing of new money, Alfonsin embarked on the most important initiative of his government, analysts here said.

If successful, the new program could break the back of the three-digit inflation that has plagued Argentina for more than a decade and vindicate a move away from the expansion of state spending and populist pump-priming that long has characterized the policies of civilian governments here.

If the government fails to win public confidence in its plan or enforce the tough new measures, the result could be economic chaos and political disaster, economic experts here said. In the past, they noted, attempts by Argentine governments to implement such sweeping measures as price controls invariably have been frustrated.

"The costs of failure are easy enough to imagine," commented the Buenos Aires Herald. "Inflation would turn to hyperinflation, the recession would get far worse . . . and sooner or later, the government would fall, to be replaced by some sinister and brutal dictatorship."

Alfonsin's move toward new policies has come amid widespread discontent with the government among Argentina's traditional power centers: the business establishment, industrial unions, the Roman Catholic Church hierarchy and the military.

Some leaders of Alfonsin's Radical Civic Union blame these conditions on adverse international economic factors or resistance by powerful sectors to democracy. Others concede that the government's inexperience and failures in its political strategy brought on much of the trouble.

"We were a new team and like any new team we were idealistic," said one presidential adviser, who asked not to be named. "This is a government that has learned on the job, and we have learned that you can't change everything from night to day."

Alfonsin originally focused on political reform, hoping to cut down the traditional power of elites in the military and unions that have dominated Argentine politics since the 1940s. But attempts at sweeping changes in both areas were blocked last year and antagonized both institutions.

While military opposition has remained muffled, and mostly passive, the bureaucratic establishment of the General Confederation of Labor, the national union confederation, has now become the de facto leadership of antigovernment mobilization, mounting two national strikes on quasi-political grounds.

Government officials concede that major economic failures increased the unions' following. With little experience or technical support in economic management, Radical party policy-makers erred in trying to treat Argentina's stagflation by stimulating consumption, a policy that had proved successful briefly during the last Radical administration in the 1960s.

As the government stubbornly increased real wages last year, inflation soared from an annual rate of 450 percent in December, 1983 to more than 700 percent by early this year. Meanwhile, Alfonsin became enmeshed in a costly confrontation with the International Monetary Fund and banks, erroneously betting that the burden of Argentina's foreign debt could be resolved by obtaining a "political solution" from creditors.

While the mounting economic crisis all but forced the abandonment of these policies, Alfonsin's political character allowed broad latitude for change. A relative maverick within the Radical party until he swept to his presidential victory in 1983, the president played a minor role in the previous Radical administration and is less bound to its traditional establishment and ideology.

In addition, Alfonsin has maintained his position as Argentina's strongest popular leader, drawing on a base that seems to respond to him alone. While recent polls here show a sharp plunge in support for the Radicals in upcoming parliamentary elections, the president's approval rating remains above 60 percent.

While the government's new shock program and accompanying agreement with the International Monetary Fund embody the short-term economic policy, other measures have signaled a more far-reaching break with the past.

While visiting Houston in March, Alfonsin unveiled a new policy for investment by foreign oil companies in Argentina, virtually reversing the Radical party position on a controversial political issue. Government officials say that step was only the beginning of a drive to open up a traditionally protectionist and sometimes xenophobic country to new investment and technology.

Similarly, Alfonsin has moved since February to replace loyal Radicals in several key government ministries with respected professionals bearing few political credentials.

This forced updating of state and party policy has been accompanied by a moderation of the Radicals' strong initial drive to restructure the unions and military. Although Alfonsin's recent attempts at conciliation with union, military and church leaders have seemed to bring few results, they have shown his willingness to accept the power of such institutions temporarily rather than seeking to confront them at a time of economic crisis.

The government also has taken a more pragmatic approach to relations with the Reagan administration, forgoing the well-established practice by civilian Argentine governments of resisting any regional initiative by Washington. Alfonsin, who denounced the U.S. invasion of Grenada during his election campaign, described President Reagan's recent peace plan for Nicaragua as "a positive element."

The most telling indication of the ground-breaking nature of these shifts, and the extent of potential resistance to them, has been the emergence of opposition within Alfonsin's party. As the president promised austerity, a group of Radical congressman issued opposing calls for economic expansion and new demands on creditors for drastically revised payment terms for the foreign debt.

Other government and party factions have strongly resisted Alfonsin's new petroleum policy and conciliatory moves toward unions.

Many politicians believe this factionalism, combined with strong resistance from unions, could spell the failure of Alfonsin's new policies. But others here believe the president has adopted the best possible course in his time of crisis.

"In the back of the dark tunnel there is a light," said writer J.M. Vera in a recent analysis in the newspaper Clarin. "It has been lit by the president by recognizing errors, ordering changes and reiterating his willingness to accept his responsibility . . . in the transformation from politician to statesman."