estor Kirchner, who steered Argentina out of crisis and political instability with a leftist populism that thrilled the poor and exasperated the wealthy, died suddenly of a heart attack Wednesday with his wife, President Cristina Fernandez, by his side.

His death, at 60, abruptly ends a plan the couple had to keep succeeding each other and holding onto power for many years. With next year’s elections looming, Fernandez will have to run for re-election without her closest adviser, the charismatic party leader who kept a tight lid on the country’s unruly political scene.

Kirchner had a history of heart trouble, undergoing emergency surgery on his carotid artery in February and an angioplasty in September, but refused to slow down, campaigning daily to lay the political groundwork for another run at the presidency by him or his wife.

He suffered another heart attack early Wednesday and was pronounced dead at 9:15 a.m. after efforts to revive him failed, a presidential spokesman said.

The news shocked Argentines, who turned out by the thousands in Kirchner’s honor, filling the Plaza de Mayo outside the presidential palace Wednesday night.

“He’s someone who for the first time in our democracy, turned his politics toward the workers and the people. That’s why so many are here. The plaza shows that the people will support and deepen his model,” said Juan Pablo Mazzieri, 39. Fernandez, he added, “has the capacity to go it alone with all the people’s support.”

But Kirchner’s death leaves a gaping hole in Argentine politics.

While Fernandez is a powerful figure in her own right, Kirchner was seen as the heir to Argentine Gen. Juan Domingo Peron, the legendary strongman whose advocacy for workers brought generations into the middle class. Also like Peron, he tolerated few challengers, keeping in check the nation’s labor unions, activist groups, governors and mayors — political players who move thousands of voters and whose allegiance is vital to maintaining public order.

One of Kirchner’s latest campaign promises was to support a labor movement effort to require all large businesses to open their books to the unions and turn 10 percent of their profits over to the workers. Giving them half the profits would be better still, he suggested at a political rally.

“After Peron and Eva Peron, nobody has done so much for the workers as Nestor Kirchner,” said Hugo Moyano, Argentina’s most powerful union leader, who now doubles as a Peronist party leader in Buenos Aires province.

Kirchner was governor of a thinly populated southern state when he was pulled from relative obscurity to become a presidential candidate in 2003, a time when Argentina was struggling to emerge from a devastating economic crisis. He captured just 22 percent of a first-round vote despite the outgoing president’s support, and took office after his rival then dropped out.

Within just a few years, he had reestablished Argentina’s all-powerful presidency and become a major figure in Latin American politics, abandoning the “Washington consensus” of tight fiscal policies and free trade, isolating the country from foreign debt markets and imposing stringent controls on the flow of money and goods in and out of the country.

Argentina’s economy grew by more than 8 percent a year during his presidency, enabling him to cancel most of the country’s world-record debt default and pay off $9 billion in loans from the International Monetary Fund, whose guidance he blamed for ruining economies around the globe.

Then, at the height of his popularity in 2007, he stepped aside, enabling his wife to succeed him and setting the stage for what many hoped or feared would be a leftist dynasty in which husband and wife would take turns as president on into the future, sidestepping constitutional limits on re-election.

At the time, Kirchner joked that he would do nothing more than hang out in a literary cafe. Instead, he and his wife worked together to increase their hold on Argentine politics. Thanks to their skills, Fernandez has been able to rule by decree for much of her presidency, despite losing majorities in Congress in midterm elections.

Without her husband, Fernandez is likely to face new threats from the left and right.

But Moyano, the nation’s most powerful union leader, quickly fell into step, ordering an emergency meeting of the General Labor Confederation, or CGT, where he said union leaders would “express our total support for the tenure of Cristina Kirchner.”

Leftist activists also fell in behind the president. “We’ll be demonstrating in the streets that we are millions who will replace Kirchner,” said the leader of the Evita Movement, Emilio Persico.