Could Donald Trump make a comeback? Will anyone in Russia challenge another six years for Vladimir Putin?

With half the world heading to the polls in 2024, and some 30 countries electing a president, here are five key elections to watch:

US: Trump-Biden rematch?

On November 5, tens of millions of Americans will choose the 60^thpresident of the United States in a contest that could keep incumbent Joe Biden in power until the age of 86.

Poll after poll shows that a majority of voters think the gaffe-prone Democrat is too old to be commander-in-chief, despite his likely rival, ex-president Donald Trump making similar slip-ups at 77.

Disinformation looks set to be a feature of the campaign, a hangover from the last foul-tempered contest which ended with Trump supporters storming the US Capitol to try to halt the certification of Bidens victory.

Trump goes into the Republican party nomination contest the standout favourite, despite multiple criminal trials hanging over him.

Russia: Putin eyes six more years

Vladimir Putin has been Russias leader for the past 23 years. In 2020 he had the constitution amended to allow him to theoretically stay in power until 2036, which could potentially see him rule for longer than Joseph Stalin.

With the war in Ukraine used to lock up or silence dissenters and opponents, there is little chance of anyone standing in the way of him securing another six years in the March election if, as expected, he decides to run.

His long-time nemesis Alexei Navalny is serving a 19-year sentence in a harsh penal colony.

Another potential challenger, Ukraine war veteran and nationalist blogger Igor Girkin, has announced his intention to run but he is locked up too, for an indefinite period.

India: Modis great power play

Nearly one billion Indians will be called on to vote in April-May when the worlds most populous nation goes to the polls in an election in which Prime Minister Narendra Modi and his nationalist BJP party are seeking a third term.

Modis political career and success have been based on support from Indias one-billion-plus Hindus and, critics say, stoking enmity toward the countrys large Muslim minority.

Despite a crackdown on civil liberties on his watch, he goes into the vote the clear favourite, with his supporters crediting him with boosting his countrys standing on the global stage.

India in August became just the fourth nation to land an unmanned craft on the Moon after Russia, the US and China, and plans to send a man to the Moon by 2040.

European Union test for populists

The worlds largest transnational election in June will see more than 400 million eligible voters from 27 European Union countries pick 720 European Parliament members that decide on issues ranging from mobile phone roaming charges to the privacy of online data.

The vote will be a test of support for right-wing populists, who have the wind in their sails after the victory of Geert Wilders anti-Islam, anti-EU PVV Freedom Party in recent Dutch elections and last years win for Giorgia Melonis far-right Brothers of Italy.

Hailing Wilders win, French far-right National Rally MEP Jordan Bardella wrote on Facebook: Bring on June 2024!

Mexico: First Mexican woman president?

A leftist former mayor of the capital and a businesswoman with Indigenous roots are both vying to make history in Mexico in June by becoming the first woman president of a country with a tradition of machismo.

Former Mexico City mayor Claudia Sheinbaum is running on behalf of outgoing President Andres Manuel Lopez Obradors Morena party.

Her outspoken opponent Xochitl Galvez has been selected to represent an opposition coalition, the Broad Front for Mexico.

A young governor from Nuevo Leon state, Samuel Garcia, also recently joined the race.

Early polls show Sheinbaum enjoying a strong lead. (AFP)