For weeks the sky over Mexico City has kept its inhabitants guessing – one moment it has been azure and sunny; the next an ominous grey vault of clouds belly full with rain. The downpours, when they come, whip flash floods across streets and topple trees.

“At my age you learn to be ready for anything,” said Mario Rojas, 86, clutching an umbrella and tramping in rubber boots across Avenida Alvaro Obregón to his tailor’s shop. He wasn’t referring to the weather. “This vote, God knows what happens after.”

Mexico elects a new president and congress on Sunday, after a fraught campaign that has shone a harsh light on its fledgling democracy. Whoever wins the keys to Los Pinos, the presidential mansion, will inherit a country as uncertain as its current weather and a people demoralised by the seemingly never-ending “war on drugs”.

Polls suggest that the conservative Institutional Revolutionary party (PRI), which ruled through patronage and repression from 1929 to 2000, will reclaim power with the telegenic Enrique Peña Nieto – a stunning comeback for a movement once consigned to history’s dustbin. “We will reduce poverty and regain peace and security in the whole country,” he told a final rally.