[tom bombariffic]

There's no way of telling what would have happened if Sam had had to finish the quest on his own, but my speculation is that he'd probably cast the ring into the fire. (I'm taking it that he'd get there for the purposes of this further speculation; frankly Im not sure he actually would.) The reason that frodo and bilbo are able to resist the power of the ring for so long is that hobbits have such little craving for power, and it takes less of a hold on them.

Sam is depicted as the quintessential hobbit in the book, and, while he may not have understood the urgency of the quest, I think he would resist the power of the ring even better than Frodo. Add that to the fact that he'd only be bearing it for a very short time (comparatively), and I think he'd still be able to cast the ring away.

But Tolkien had built up the story of Frodo fighting the ring, trying to resist its power, from the start of the journey. Sure, in the final showdown in mt Doom, The Ring beats Frodo, but the result goes right to the wire, as it were - the tension builds up to a terrific climax as he stands in front of the fire. To kill Frodo off and pass the ring to Sam in a tag-team style manouvre would ruin this effect, and the personal tussle between ring and bearer would be lost, and start over again. Sam may cast the ring into the fire, but the sense of climax and achievement would be lost, and the actual power of the ring possibly underestimated - it is only when Frodo cannot throw the ring in that we realise how entirely it posesses him.