MANILA, May 12 (Reuters) - Voting was underway in the Philippines on Monday for a normally low-key midterm election that is showcasing an emotionally charged proxy battle between President Ferdinand Marcos Jr and ally-turned-foe, Vice President Sara Duterte.

Duterte and Marcos are not on the ballot for more than 18,000 positions but have been campaigning aggressively for their slates of candidates, as their bitter rivalry dominates a contest that could influence the future dynamic of power in the country of 110 million people.

At stake for Marcos is his policy agenda, his legacy and influence over his succession in 2028, while Duterte's political survival could ride on the vote, with an impeachment trial looming that could kill any future hopes of becoming president and following in the footsteps of father Rodrigo Duterte.

While posts for mayors, governors and lower house lawmakers are up for grabs, the tussle for Marcos and Duterte is over a dozen coveted seats in the 24-member Senate, a chamber with sweeping legislative influence and political clout that can shape public opinion and sink presidential ambitions.

"This election is more than an informal referendum on the Marcos administration," said Aries Arugay, a political science professor at the University of the Philippines.

"The Senate race is the key proxy battle ... Marcos needs to retain a majority, or supermajority, to push forward his legislative and economic agenda."

Photo from Reuters