Corruption and political vendettas have convulsed the nation in recent years, and the country has cycled through four presidents and two congresses in the past five years.
Perhaps most critically, Mr. Castillo, who, who has never held office, lacks the political experience and popularity that buoyed other left-wing leaders who took power in South America.
“As a political figure, he has a lot of problems that lead to instability,” said Mauricio Zavaleta, a Peruvian political scientist.
In Bolivia in 2005, Evo Morales, who became the country’s first Indigenous president, won in the first round with more than 50 percent of the vote, he pointed out. In Venezuela in 1998, Hugo Chávez “was an electoral storm.” Luiz Inácio Lula da Silva, in Brazil in 2002, and Rafael Correa, in Ecuador in 2006, were established figures first elected president by wide margins.
“Castillo is not part of those phenomena,” Mr. Zavaleta said.
Moreover, he said, Mr. Castillo is unlikely to have the support of Congress, the military, the media, the elite or a large political movement. “He simply doesn’t have the muscle to carry out the ambitious reforms he’s proposed,” Mr. Zavaleta said.
Mr. Castillo has promised to overhaul the political and economic system to address poverty and inequality, and to replace the current Constitution with one that would increase the state’s role in the economy. He campaigned wearing a traditional farmer’s hat, and sometimes appeared on horseback, or dancing with voters.