But should Moreno be forced into a second round, analysts expect Ecuador's fractured opposition to coalesce around Lasso amid an economic downturn and corruption scandals in OPEC's smallest member state.
That would further bolster the right in South America, after Argentina, Brazil and Peru all shifted away from leftist rule in the past 18 months as a commodities boom ended.
Still, Ecuador's ruling Country Alliance party remains popular with many poor in the country that is home to volcano-topped Andean plateaus, lush jungles and the Galapagos Islands.
"This government must continue because it's the best one in all political history. It's built everything: Hospitals, roads, schools," said student Cristopher Gonzalez, 26.
"I don't believe any candidate but Lenin Moreno."
Moreno, who lost use of his legs two decades ago after being shot during a robbery, has a more conciliatory style than the fiery Correa and has promised benefits for the disabled, single mothers, and the elderly.
But the economy is weighing heavily on voters.
Unemployment is running high, the middle class is upset over tax hikes, and corruption scandals at state-run oil company PetroEcuador and Brazilian conglomerate Odebrecht have outraged many.
"We've lived 10 years of disgrace, 10 years of theft, corruption, false honesty, a country-wide regression where only a few have benefited," said Angel Mendoza, a 45-year-old architect who was supporting Lasso.
Lasso has campaigned on a platform to revive the economy, which is dependent on exports of oil, flowers, and shrimp, by slashing taxes, fostering foreign investment, and creating 1 million jobs in four years.
He has also vowed a clean break with Correa's foreign policy. He would remove Wikileaks founder Julian Assange from the country's embassy in London by late June and take a firm stance against Venezuela's socialist government.