Thousands of shocked people have gathered to pay tribute to former Argentina president Nestor Kirchner, who has died suddenly of a heart attack.
His wife and current president Cristina Fernandez now faces the prospect of running for re-election without her closest adviser.
Mr Kirchner, 60, who steered Argentina out of crisis and political instability with a left-wing populism that thrilled the poor and exasperated the wealthy, died with his wife by his side.
His death abruptly ends a plan the couple had to keep succeeding each other and holding on to power for many years.
With next year's elections looming, Ms Fernandez will have to run for re-election without the charismatic party leader who kept a tight lid on the country's unruly political scene.
Mr 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 on Wednesday and was pronounced dead at 9.15am local time after efforts to revive him failed.
Shocked Argentines filled Buenos Aires' Plaza de Mayo outside the presidential palace.
"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.
Ms Fernandez, he said, "has the capacity to go it alone with all the people's support".