The Stochastic Man by Robert Silverberg
Lew Nichols is in the business of stochastic prediction. A mixture of sophisticated analysis and inspired guesswork, it is the nearest man can get to predicting the future. And Nichols is very good at it. So good that he is soon indispensable to Paul Quinn, the ambitious and charismatic mayor of New York whose sights are firmly set on the presidency. But there is nothing paranormal about stochastic prediction: Nichols can't actually see the future. However, Martin Carvajal apparently can and he offers to help Nichols do so too. It's an offer Nichols can't resist, even though he can clearly see the devastating impact that knowing in advance every act of his life has on Carvajal. For Carvajal has even seen his own death.