Opinion polls in the European Union's poorest country, where the average monthly salary is just 500 euros ($540) and corruption is rife, indicate a tight race and a strong showing by nationalists.
The karate-kicking Borisov's enthusiastically pro-EU GERB party and the Socialist Party (BSP), newly led by the energetic Kornelia Ninova, are both seen garnering around 30 percent.
But in the ex-communist nation's third election in four years, many voters are turning away from the main parties towards groups on the fringes, or are not bothering to vote.
"I will back neither Borisov nor the opposition Socialists. I do not believe them any more," teacher Tsvetomira Tosheva, 47, told AFP in Sofia.
Borisov, 57, once a bodyguard for Bulgaria's last communist leader, has long been the dominant figure in national politics, serving as premier from 2009 to 2013 and again from 2014 to 2017.
In between the BSP was in power for barely a year.
Both times Borisov quit early, first in 2013 after mass protests and then last November after his candidate for the presidency was beaten by a air force general backed by the BSP.
