Pedro Briones, leader of the left-leaning Revolucion Ciudadana party of ex-President Rafael Correa, was shot to death near the northwest province of Esmeraldas on Monday.
Front runner in Sunday's presidential election and member of the same party, Luisa Gonzalez, confirmed the fatal shooting.
Following the assassination of center-right presidential candidate Fernando Villavicencio last Wednesday, Ecuadorian President Guillermo Lasso announced a state of emergency across the country for 60 days.
"I am declaring a state of emergency for 60 days [...] From now on, the armed forces are mobilized throughout the country to guarantee the security of citizens, the tranquility of the country and free and democratic elections," President Lasso said in an address to the nation through state media.
The number of political assassinations has increased to three in the last four weeks, with one involving a presidential candidate.
"Ecuador is experiencing its bloodiest era," Gonzalez wrote on Twitter, now X, adding: "We owe this to the total abandonment of an inept government and a State taken over by the mafias."
Diana Atamaint, the president of the National Electoral Council of Ecuador, said on Thursday that the date of Ecuador's early presidential election scheduled for August 20 would remain unchanged despite the assassination of Villavicencio.
The country's armed forces will reportedly guarantee the security of Ecuadorians throughout the planned electoral process, adding that the protection for the remaining presidential candidates would be doubled, media reported.
The Organization of American States (OAS) has called on the Ecuadorian government to conduct a thorough investigation and strengthen security in the country after the incidents.