His only previous race was a losing bid for Texas attorney general in 1978, but former Secretary of State James A. Baker III is leaning ever closer to a run for the 1996 Republican Presidential nomination. Associates say Baker thinks Bill Clinton is vulnerable. Besides, no other challenger seems to be catching fire with the GOP faithful. At 64, Baker senses that it's now or never. Says one friend: "He hates the thought of losing, but he hates even more the thought of never achieving his ambition to be President."