By JUAN FORERO and LARRY ROHTER

TIWANAKU, Bolivia, Jan. 21 - When Evo Morales, an Aymara Indian and former head of the Bolivian coca growers union, is sworn in as president on Sunday, it may be the hardest turn yet in South America's persistent left-leaning tilt, with the potential for big reverberations far beyond the borders of this landlocked Andean nation.

While mostly vague on details, and recently moderating his tone, Mr. Morales promises to transform Bolivia and "end the colonial and neoliberal model," as he put it on Saturday in an elaborate ceremony at the sacred ruins of this pre-Incan civilization.

He has said he would "depenalize" cultivation of coca, the prime ingredient for cocaine, which Washington has spent hundreds of millions of dollars and more than two decades trying to eradicate.

He pledges to inject the state in Bolivia's oil and natural gas industry, troubling the multinational energy companies that first flocked here in the late 1990's, even though Mr. Morales recently said he would not expropriate foreign holdings.

He has disparaged American-backed free trade policies, and seems certain to stand as the southernmost outpost of a new anti-American nexus with Cuba and Venezuela, whose president, Hugo Chávez, has become among the Bush administration's most ardent critics.

Any and all of those steps in a country where coca tracts and rich energy holdings give it a strategic importance far outweighing its tiny population could unsettle Washington and the region.

Bolivia's gas reserves, the continent's second-largest, help power South America's largest economies. Brazil has plowed $1.5 billion into energy investments in Bolivia and worries about rising drug and crime problems in its urban slums if Bolivia's coca crop is not controlled.

Mr. Morales is at least the seventh Latin American leader to take power since 2000 from the left, a varied crop that ranges from Chile, Brazil, Argentina, Uruguay and Ecuador to Venezuela, with strong leftist contenders surging in Peru and Mexico, both of which will also hold elections this year.

His success is also the most prominent example of Latin America's recent democratic revolutions. Throughout the region, the indigenous and the poor, increasingly mobilized by frustration with Washington-backed economic prescriptions, have used the ballot box to put in place a group of leaders more representative of their interests for the first time in nearly five centuries. With the exception of Mr. Chávez, who is bankrolled by Venezuela's oil wealth, most of the continent's other left-leaning leaders, like Luiz Inácio Lula da Silva of Brazil, have pursued pragmatic policies once faced with the real task of governing.

In recent weeks, Mr. Morales has toned down some of his more strident language and struck a more accommodating note with American officials. But in Bolivia's case, political analysts here say, it is far harder to know exactly how Mr. Morales might rule.

Mr. Morales, a former congressman, is untested as an executive and known less as a pragmatist than as a fiery orator and protest leader. Several of his associates, including Vice President-elect Álvaro García and Carlos Villegas, who will oversee economic planning, are leftist academics with no experience in government.

"There could be realism and pragmatism in their policies, or they could allow ideology to guide them," said Roberto Laserna, a political analyst with San Simón University in Cochabamba, the city where Mr. Morales makes his home. "But we do not have a way to gauge their management experience."

What is clear is that Mr. Morales's compelling storybook rise to power has brought this isolated country of nine million people the kind of international attention it has rarely received. A former llama herder who saw four siblings die in childhood, Mr. Morales won Bolivia's Dec. 18 election in a landslide not seen since the country's return to democracy in 1982.

The first Indian elected president in a country where most people are indigenous, Mr. Morales, dressed notably casually in an open-collar shirt and sweater, embarked on a 10-day world victory tour. He met this month with the President Jacques Chirac of France, President Hu Jintao of China and Prime Minister José Luis Rodríguez of Spain.

On Saturday, in a ceremony attended by tens of thousands of Aymara and Quechua Indians at this archaeological site some 14,000 feet above sea level, Mr. Morales donned the replica of a 1,000-year-old tunic similar to those once used by Tiwanaku's wise men, was purified in an ancient ritual and accepted the symbolic leadership of the myriad indigenous groups of the Andes.

"We are not alone," Mr. Morales told the crowd. "The world is with us. We are in a time of triumph, a time of change."

On Sunday, for his official state inauguration, he expected about a dozen foreign leaders, far more than have ever attended a Bolivian inauguration.

Part of that solidarity stems from his role as representative of a new Latin American pole in global politics, as the region coalesces as a counterpoint to unpopular United States policies. More and more Latin American countries are taking exception to Washington's economic prescriptions and those of the International Monetary Fund. Some are strengthening ties with China, which is investing heavily in the region.

Many have refused to go along with Bush administration demands to exempt Americans from criminal prosecutions at the International Criminal Court at The Hague. No South American countries have sent soldiers to support the war in Iraq. And anti-American criticism has become political sport, as opinion surveys give President Bush the lowest standing in Latin America of any American president in the region's history.

As varied as the region is, no other part of the world has seen as uniform a shift in its political landscape.

Mr. Morales, however, has already had to find middle ground between the explosive populist talk that helped propel him to power and a pragmatic path that will help Bolivia's tiny $9.5 billion economy grow, said Nancy Birdsall, president of the Washington-based Center for Global Development, which studies economic issues affecting developing countries.

In a 2002 interview, Mr. Morales told The New York Times that the solution to Bolivia's economic troubles was "communal socialism," having peasant communes run mineral and metals mines and agriculture. Mr. Morales, while railing at globalization, now says that trade agreements can work, if fair to both sides, and that foreign investment is needed.

Since calling Mr. Bush a "terrorist" in December, he has sounded a more conciliatory tone. He noted this week that he, too, had been the target of harsh barbs from American officials.

"Everything here is pardoned," Mr. Morales said. "We are in new times. Let us start talking, not in a dialogue of submission, but to find solutions."

Among those who will advise Mr. Morales is Juan Ramón Quintana, a moderate academic who has worked extensively with international organizations and governments. "There is still the perception of Evo Morales as a radical leader," he said in an interview. "But Evo Morales is undergoing an important transformation. We all believe that he can become a statesman."

The United States, too, has been more accommodating. Thomas Shannon, the assistant secretary of state for Western Hemisphere affairs, recently told reporters that Washington wanted dialogue with Mr. Morales and to continue "positive relations" between the two countries.

On Saturday night, he and the American ambassador, David Greenlee, met with Mr. Morales to lay the groundwork for future discussions. "We want the Bolivian people to succeed, and for the Bolivian people to succeed, this government has to succeed," Mr. Shannon told reporters after the meeting. "It's a democratically elected government, and we hope that we're going to be in a position to work with them."

Still, if the Bush administration decides that Mr. Morales is pursuing policies that run counter to American interests, either in trade or on drug policy, aid could be frozen or cut - and the United States is this country's largest donor. It has provided $655 million from 2000 to 2004, two-thirds of it for development, and is considering a Bolivian request for $600 million to build roads.

Solidarity aside, for Bolivia's neighbors, too, Mr. Morales's ascension creates new challenges as well that are hardly mitigated by the fact that most are led by nominally leftist governments. The main concern is assuring access to Bolivia's vast natural gas reserves.

More than any other country, Brazil has vital economic interests to protect. About half the natural gas consumed in Brazil comes from Bolivia, one third of Bolivia's exports go to Brazil, and Brazilian companies are the largest group of investors in Bolivia, led by the state-dominated oil company Petrobras.

So Brazil has watched with concern as Bolivia has become increasingly unstable in recent years. President da Silva's government has been careful not to arouse Bolivian nationalism, reacting calmly to Mr. Morales's statements that he is seeking "partners, not bosses" in developing its gas reserves.

When Mr. Morales visited Brazil this month, Mr. da Silva invited Bolivia to become a full member of Mercosur, the South American trade union that Brazil dominates. Membership, some analysts in the region say, could constrain any Bolivian temptation toward radicalism.

"We have the advantage, in that we are not run by a Brazilian Bush but a president whose origins, like those of Morales, are in the labor movement and who wants to avoid excessively dramatic conflicts," said Helio Jaguaribe, a leading Brazilian foreign policy analyst.

The stakes are high for Argentina, too. Until now, Bolivian governments have provided it with gas at below market prices. Mr. Morales said during a visit to Buenos Aires this week that he planned to end that arrangement, a step that is likely to add inflationary pressures.

But Bolivia's most prickly relations will be with Chile, even with the election last week of the Socialist Michelle Bachelet as president. Chile's victory in a 19th-century war that cost Bolivia its coastline continues to bedevil the relationship. Mr. Morales has even criticized Chile's capitalism and close relations with the United States, and anti-Chilean sentiment remains rampant here.

"The United States wants to convert Chile into the Israel of Latin America," Mr. Morales charged in a newspaper interview that has been widely quoted in Chile. The two countries have not had full diplomatic relations since the 1970's, and Mr. Morales led a campaign against a pipeline that would have sent Bolivian gas to Mexico and the United States via a Chilean port.

Still, Mr. Morales has invited Chile's departing president, Ricardo Lagos, to the inauguration, the first Chilean leader to visit Bolivia for such an occasion in decades. Ms. Bachelet has said she favors greater integration between Chile and its poorer neighbors.

Indeed, Mr. Morales begins his term with good will - and wariness - all around. Venezuela has pledged diesel fuel and energy cooperation, while Spain has offered debt relief, according to Bolivian officials. Other countries have pledged stronger ties.

Mr. Morales, while welcoming trade with countries as far away as Belgium and South Africa, has been besieged by requests from countries with investments here to ensure that Bolivia maintains a healthy environment for foreign companies.

Mr. Morales has tiptoed around them so far. But there is little doubt he will also give more authority to the state, remaking some ministries and creating others. "The state needs to be the central actor to plan economic development," he explained.

Juan Forero reported from Tiwanaku for this article, and Larry Rohter from Buenos Aires.