Only two months ago, many were wondering if the 57-year-old former Israeli chief of staff and leader of the Labour party stood a chance. He showed political inexperience, and Israeli politics are full of minefields. His efforts to woo the Russian, Sephardi and ultra-orthodox voters seemed inept and were unsuccessful.
But at the last moment everything has come together. Mr Barak has shown that he has strong nerves and the capacity to plan ahead.
If elections go to the candidate who makes the least mistakes, then he will beat Mr Netanyahu hands down.
Mr Barak is not a well known character. Born in 1942 in a kibbutz south of Haifa, he spent 35 years in the Israeli army. In the last weeks his television advertising has relentlessly reminded viewers of his deeds of derring-do, such as leading seaborne commandos to kill three PLO leaders in Beirut in 1973. A general at 37, he became the Israeli chief of staff before entering politics in 1995 under the wing of the then Labour prime minister, Yitzhak Rabin.
But his military career has not always attracted good reviews. Soldiers often found him too much of a politician and politicians too much of a soldier. He was damaged by the accusation that he abandoned wounded soldiers during a training accident in 1992.
He is a shy man. He played the piano from an early age, has an interest in clocks and is well read. He avoided Israeli staff college to get a degree at Stanford University in California. He has a pinched half-smile which seems to say that he knows what a person is going to say before they say it.
As a soldier and politician Mr Barak has always been intensely competitive. In 1995 he was being groomed by Mr Rabin for ultimate leadership of the Labour party.
It seemed a long term prospect, but within a few months the prime minister was assassinated. Mr Barak became Foreign Minister.
The following year the new prime minister, Shimon Peres, lost a close- run election. Mr Barak took over the party leadership in 1997.
The omens were not good: Mr Netanyahu seemed to shrug off repeated scandals. But Mr Barak has eliminated his own weaknesses, looking to Israeli voters like an increasingly convincing alternative to their current prime minister.