Nearly two million volunteers are involved in a grassroots movement for Robredo, the incumbent vice president, as she battles to close the gap with front runner Ferdinand Marcos Junior before the May 9 polls.
Voter surveys show the son of the country's former ruler heading towards a landslide victory, the endgame of a decades-long, well-funded effort to return the powerful clan to the presidential palace they fled 36 years ago.
But there are signs Robredo's pink-colored campaign is finding traction, with huge rally turnouts, endorsements from pop stars and Catholic priests, and a bump in a recent poll raising hopes among her fervent fans.
"I'm really craving for change, for decency in the highest position," said Rocelle Mendoza, 29, who uses her own money to print Robredo T-shirts and aprons that she gives away at markets in the capital Manila.
"We are aware that we have to climb a mountain. And I think it's also why we're no longer hesitating to spend."
Some analysts have likened the feverish support for Robredo to the people-driven movement for former president Corazon Aquino in the 1986 snap election campaign that led to the ousting of Ferdinand Marcos Senior.
Robredo, a widow and former congresswoman, made a last-minute decision to enter the presidential race in October after Marcos Jr declared his candidacy.
That set up a rematch with her rival in the 2016 vice presidential contest that she narrowly won.
A volunteer-driven campaign mushroomed across the country, putting the mild-mannered Robredo in second place in the polls.
But the chances of beating Marcos Jr again appear slim.
Relentless attacks from President Rodrigo Duterte and a nasty misinformation campaign on social media have undermined Robredo's popularity and eroded her support among local officials, who are key to garnering votes.
The latest poll by Pulse Asia Research shows her gaining ground, but Marcos Jr still leads by 32 percentage points.
"It remains a relatively tall mountain for Robredo to climb and is still Marcos's race to lose," said Eurasia Group analyst Peter Mumford.
'We won't stop'
Marcos Jr has been bolstered by an alliance with first daughter and vice presidential favourite Sara Duterte, the backing of powerful political clans, and a massive social media campaign portraying his family in a positive light.
But Robredo's loyal followers are undeterred, dipping into their own pockets to support the former lawyer for farmers and battered women in an election they see as critical for the country's future.
After six years of authoritarian firebrand Duterte and his deadly drug war, Robredo volunteers say they are scared of a Marcos Jr presidency, pointing to the human rights abuses committed during his father's 20-year rule.
"I'm afraid there will be a repeat of martial law. The videos I see about torture and rape, it's scary," said Sheilla Oledan, 31, as she door-knocked in suburban Manila.
So far, nearly two million volunteers have signed up to campaign for Robredo on social media and on the ground, said Georgina Hernandez Yang, joint leader of a group set up by the Robredo camp to coordinate volunteer efforts.
They belong to groups representing diverse interests and ages, such as "Seniors for Leni", "Doctors for Leni", and "Youth Vote for Leni".
Talking to people face-to-face was the best way to "break out of our echo chambers" on social media, said Yang.
"There's a lot of fake news and disinformation that we're able to clarify and correct."
But changing minds is not easy.
In suburban Manila, supporters hand out pink flyers and face masks to residents and street vendors, striking up conversations to explain Robredo's platform.
Some people wave them away while others listen to their spiel. Occasionally, they are heckled or harassed.
"Someone asked us to kiss them first before they will listen. It's very offensive," complained Marielle Chico, 26.
After speaking to a Robredo volunteer, rice and egg vendor George Tolentino said he was still undecided.
"I will study first their agenda," the 55-year-old said.
As election day looms, volunteers like Mendoza are determined to continue campaigning.
"It all comes down to May 9," she said defiantly.
"We won't stop."