The president's loyalists are poised to sweep the National Assembly elections Sunday, adding to the litany of woes facing his chief rival, Juan Guaidó.
The former president crossed the border from Argentina and was welcomed by a large group of supporters, just a day after his former economy minister, Luis Arce, was sworn in as president.
After mass protests, and amid a pandemic, Chileans go to the polls Sunday for a referendum over whether to scrap the constitution introduced under Gen. Augusto Pinochet's rule.
Luis Arce, a candidate handpicked by former President Evo Morales, is expected to be confirmed as the winner in the country's most important presidential election in a generation.
Pandemic emergency aid gave an unlikely ratings boost to President Jair Bolsonaro, who has criticized welfare and protested virus prevention measures. Now he's weighing further social spending.