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.
- 'Second-class member' -
If Ninova can become premier this raises the prospect of NATO member Bulgaria, which has long walked a tightrope between East and West, drifting more towards Moscow.
Ninova has said she is not content with Bulgaria being a "second-class member" of the EU and that she will veto an extension of EU sanctions on Russia.
Russia, with which Bulgaria has long had close cultural and economic ties, has been accused of seeking to expand its influence in other Balkan countries in recent months.
But Borisov has also said that he wants more "pragmatic" ties with Russia and Ninova, 48, insists that she remains committed to the EU.
"We are the party that ushered Bulgaria into the European Union and NATO and we stand by (our obligations in) these organisations," she told AFP in a recent interview.
Bulgaria is home to a 700,000-strong Muslim minority, most of them ethnic Turks, while at least 200,000 ethnic Turks with Bulgarian passports live in Turkey.
Turkey's support for a new party, Dost, which unlike the MRF fervently backs Turkish President Recep Tayyip Erdogan, has fuelled a spat in recent days.
This has boosted the United Patriots nationalists, who blocked the border on Friday to stop voters coming in from Turkey, and who may come third.
Another potential coalition partner is Veselin Mareshki, 49, a colourful populist who likes being called the Bulgarian Donald Trump.
The resulting government may not last long.
"It seems that some configuration of political parties who support the oligarchic government model will win the elections," political analyst Evgeni Daynov told AFP.
"But because society has already realised how dangerous corruption is, this will inevitably lead to a highly unstable government."