Experts have said if Tsetska Tsacheva fails to win Bulgaria presidential election, Borisov might call early elections.
If his candidate and protegee Tsetska Tsacheva fails to win the presidency, Borisov might call early elections, plunging Bulgaria into renewed political uncertainty, experts say.
Opinion polls suggest that Tsacheva, currently speaker of parliament, will top the crowded field of 21 candidates with around 30 percent of the vote.
But close on the 58-year-old's tail will be MiG ace and former airforce head Rumen Radev, the candidate of the opposition Socialists who is seen as more sympathetic to Russia.
This will set up what surveys suggest will be a tight runoff contest between Tsacheva and Radev on November 13 to become the south-eastern European country's head of state.
While Borisov presents Tsacheva as the "mother of the nation", political scientist Dimitar Bechev warned that he had failed to grasp her "dangerous lack of charisma".
"Radev could end up carrying the day by a small margin," Bechev said.
Burly former bodyguard and police chief Borisov, 57, has injected some much-needed stability into Bulgaria since becoming premier for the second time in late 2014.
His first term ended abruptly in February 2013 when Bulgarians livid about poverty, corruption and cronyism took to the streets across the country. Eight people set themselves on fire.
More protests brought down the subsequent technocrat government after barely a year, precipitating fresh elections that returned Borisov to power at the head of a minority government.
Graft and poverty remain rife, however, and progress on reforms has been sluggish. A new voter concern in recent months has seen thousands of migrants stranded in Bulgaria.
The job of Bulgarian president is largely ceremonial but he or she is still a respected figure who chooses some top officials and can appoint technocrat governments in a crisis.
And for a country forever walking the East-West tightrope, both Moscow and Brussels will be watching closely to see whether the new president might tilt the ex-communist country more into the Russian or the Western orbit.
NATO member Bulgaria last September angered Moscow by banning Russian supply flights to Syria from using its airspace.
Rosen Pleneviev, the outgoing president, has been outspoken in his criticism of Russia and its President Vladimir Putin.
"What today Russia is trying to achieve is to weaken Europe, to divide Europe and to make us dependent," Pleneviev told the BBC in an interview published Friday.
But at the same time Bulgaria's economy is hugely reliant on Russia, particularly in gas, and the two have deep historical and cultural ties. In 2010 Borisov gave Putin a puppy.
Radev, meanwhile, told Darik radio in a recent interview: "We have lost a lot by declaring Russia more or less an enemy."
In addition he has called for EU sanctions on Russia, imposed because of Ukraine, to be lifted -- a position shared by Tsacheva until Borisov called her back into line.
Voting stations close at 8 pm (0600 GMT), with exit polls expected soon afterwards.