Bulgarians voted Sunday in a high-stakes presidential run-off that could see centre-right Prime Minister
While casting his ballot, the tough-talking premier reiterated his vow to throw in the towel if his pro-EU protegee -- parliamentary speaker Tsetska Tsacheva -- loses to former air force chief Rumen Radev, seen as friendly to Moscow.
"We will not participate in any way in the government if we lose today," Borisov said.
A political novice, the 53-year-old Radev stunned pollsters by sweeping just over 25 percent of the vote to Tsacheva's nearly 22 percent in the first round of the election on November 6.
Polling stations opened at 0500 GMT and will close at 1800 GMT, with first projections expected shortly afterwards.
The latest polls show Radev is still the favourite as voters seek to punish the government over its perceived failure to tackle rampant corruption and poverty in the European Union's poorest member state.
"I voted against Borisov because I don't think that he's honest and he hasn't really done anything to improve our lives," said 52-year-old Zora Kardachka, a dry cleaner.
Observers say the general's victory might tilt ex-communist and Soviet ally Bulgaria, which has long walked a tightrope between Moscow and Brussels, towards Russia's orbit -- a trend seen across eastern and central Europe amid rising euroscepticism.
Nearby Moldova also looked set to elect a pro-Russian president on Sunday.
Tsacheva, 58, has vowed to keep Bulgaria on a pro-European path and accused her rival of being a "red general".
But lacking charisma she has not inspired voters, in an embarrassing setback for the popular Borisov who became prime minister for the second time in 2014.
"Maybe I should have thought better when I staked the government's fate on my choice... I was wrong and I will take full responsibility," the burly ex-police chief said on Wednesday.
However, Sunday's outcome could still be swayed with support from those who had voted for other first-round candidates, observers said.
The Bulgarian president's role is largely ceremonial but he or she -- Tsacheva would be the first woman in the position -- is nonetheless a respected figure and commander-in-chief of the armed forces.
If Tsacheva loses, Borisov could announce his resignation on Sunday evening.
Fresh elections are likely to plunge Bulgaria into renewed turmoil.
Prior to Borisov's re-election, the country went through a long period of upheaval involving mass protests and a string of polls.
Despite promised reforms, graft and poverty remain rife while public anger has also been growing over thousands of migrants currently stranded in Bulgaria.
Borisov's popularity has waned and opinion polls suggest that his GERB party would fail to win an outright majority in early elections.
And Borisov appears to have badly miscalculated in nominating Tsacheva, analysts say.
"His threat to step down has mobilised his opponents more than supporters," Zhivko Georgiev of the Gallup Institute told AFP.
Russia not an 'enemy'
A win for Radev would also signal a change in direction from outgoing President Rosen Plevneliev, a strong critic of Russia.
Sofia and Moscow have deep historical and cultural ties, and Bulgaria's economy is heavily reliant on Russian energy.
Plevneliev accused Russia of trying to "destabilise Europe" by "financing anti-EU ultra-nationalists" including in Bulgaria, in an interview published on Sunday in Austrian newspaper Die Presse.
He warned that the Balkans were a tinderbox for conflict and found themselves in a tug of war between Moscow and Brussels.
"The Balkans are an important strategic part of Russian interests. Bulgaria is important for Europe because it is a key regional player. Those who seek to destabilise Europe, have the best chances of doing so in the Balkans. It's always been this way: this is where WWI was ignited."
"The (political climate) is more dangerous now than during the Cold War."
Radev, a general who trained in the United States, has repeatedly insisted that "being a member of the EU and NATO does not mean that Bulgaria must be an enemy of Russia".