Dube Egwuatu was buying a mobile telephone top-up card in an off-licence when the gunman confronted him and glared at the top, which carries an image of the Democrat US presidential candidate underneath the legend 'Believe'.
The man then launched into a tirade of racist slurs, shouting 'I f***ing hate n*****s' and urging 36-year-old Mr Egwuatu to leave the shop with him.
The man then left the shop but when Mr Egwuatu re-emerged, the attacker was waiting for him in broad daylight with a threatening-looking dog and holding a gun behind his back.
Realising what had sparked the increasingly violent assault, the terrified Mr Egwuatu zipped up his jacket to cover the image of Mr Obama and walked to his car.
But the shaven-headed man, who was white, followed Mr Egwuatu and after pulling open the passenger door pointed the gun at him.
After pleading with the man to leave him alone, the married former street warden put the keys in the ignition and turned the engine on.
The attacker then fired the gas-powered ball-bearing pistol three times, hitting the civil servant in the face, hand and shoulder.
Fearing for his life and bleeding heavily, Mr Egwuatu raced away in his car and found somewhere safe to call for help.
He was taken to hospital and later sent to have a piece of metal removed from his jaw.
Mr Egwuatu, a data analyst with Croydon Council, said: 'The venom in his voice was frightening.
'He was telling me that he was going to kill me.
'I couldn't believe it was happening - and just because I was wearing an Obama T-shirt. He was trying to make me walk somewhere quieter, saying: 'I've got something for you,' and 'I'm going to kill you.'
He added: 'Obama inspires me, his educational track record alone is quite unbelievable - that is why I was wearing the T-shirt.
'I did not think for one minute it could stir up such powerful feelings of hatred and I never said a word to him.'
Mr Egwuatu's wife, Angela, 35, said neither of them had experienced anything like it during their childhood in Nigeria.
Mrs Egwuatu, an immigration officer, said: 'At first my feelings were pure horror and now it is pure anger.
'If he had been carrying a real gun I would have been a widow. It is just ridiculous.
'I don't know how a person's mentality works. Why would a T-shirt get you to the point where you want to shoot someone.'
To the untrained eye, ball-bearing guns like the one used in the attack look every bit like a real firearm.
The potentially lethal weapons are often converted by criminals to fire real bullets, and can be bought easily in high-street shops and on websites.
The Met said it was investigating the incident, which took place in South Norwood, and that police searched a nearby house which the attacker was seen going into.
No one has been arrested.
I'm more convinced than ever the world needs to see Obama elected for so many more reasons than his detractors can produce that he doesn't.