With his close-cropped hair, bullish physique and macho image, Borisov has been one of the country's most popular figures for years, even before setting up his party in 2006.
The 50-year-old former firefighter is something of a self-made man and proud of his rise.
The son of a police officer and a primary school teacher, Borisov graduated from a firefighting and police academy in Sofia.
He then set up his own security company in 1991, providing protection for Bulgaria's ex-communist dictator Todor Zhivkov, who had been ousted in 1989.
In the mid-1990s, Borisov became the bodyguard of former boy-king Simeon Saxe Coburg, who had returned to Bulgaria after 50 years in exile.
Working for the two men helped him "comprehend history and the mechanisms of power," Borisov told AFP in an interview last week.
When the king's National Movement Simeon II party won a landslide election victory in 2001, Saxe Coburg promoted Borisov to chief of staff in the interior ministry.
His plain-speaking manner and willingness to show up at major crime scenes boosted his popularity and in 2004, he was promoted to lieutenant general, the highest possible police rank.
A year later, he quit the interior ministry to run as an independent candidate in the Sofia mayoral elections, which he won.
He was re-elected as mayor of the capital for another four years in November 2007, but will have to step down if appointed premier.
His time as mayor so far has been marked by a failure to fulfil election promises of solving Sofia's garbage collection problems and incessant quarrelling with Prime Minister Sergey Stanishev and his socialist administration.
Even after swapping his general's uniform for a politician's suit, Borisov kept his heartbreaker ladies man's image, which won him the title Man of the Year twice -- in 2002 and 2006.
He was able to set up his opposition party largely because of his popularity as a sharp critic of the current administration, which made people feel he was one of them, analysts said.
A strong economic team headed by a World Bank analyst and a programme of anti-crisis measures has further added to his and the GERB's popularity, the analysts said.
While GERB won the most votes on Sunday, well ahead of Stanishev's socialists -- who won just 17.1-18.5 percent according to exit polls -- the lack of an outright majority will force Borisov to seek allies to form a coalition government, even if he is openly sceptical of power-sharing deals.
In the interview with AFP, Borisov said he "needed a majority to push through the radical changes needed to change the status quo."
A partner, with only a small number of parliamentary seats, could try to blackmail their bigger ally, he argued.
Maintaining his credentials as man-of-the-people and former top crime-buster, Borisov pledged to purge Bulgaria of corruption, restore Brussels' trust in its poorest newcomer and convince the European Commission to unfreeze millions of euros in subsidies to the country, withheld last year over fraud concerns.
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