What ingredient in tobacco that is the main carcinogen?

None of them. Epidemiological studies have SUGGESTED that tobacco MAY increase the RISK of developing cancer. Barbaric laboratory experiments on rats and dogs (try Googling "beagles smoking") have provided some further supporting EVIDENCE (and also contradictory evidence). Wouldn't the billions wasted be better spent researching a cure for cancer? Smoking rates are decreasing yet the incidence of cancer is increasing.