By Richard Sammon, Senior Associate Editor May 7, 2008 If there's one word to describe the driver of this presidential race, it may not be race, gender, age, money, Iraq or economy. The most powerful word arguably is "proportional," referring to the Democratic Party rule on dividing delegates based on primary results, not awarding them on a winner-take-all basis. Candidates have lived and perished by it ... Sure, this sounds technical, but sometimes a technical rule defines the entire race, the ultimate outcome and its impact on history. The Democratic Party's rule on dividing delegate votes on a proportional basis is why Barack Obama is near to locking up the nomination. It's why Clinton has not been able to break his dominance in the delegate math. Even with her wins in Ohio, Pennsylvania and Indiana, the delegates are nearly evenly split between the two, making even small net gains difficult. The only way for her to close ground substantially has been to win with super-large margins, such as 70 to 30 or so. With two very good candidates after others bowed out, that's proven impossible. It's also the reason Obama has labored under criticism that he's not "closing the deal" even as he gets closer. Had Democrats opted for a winner-take-all approach, such as Republicans have, Clinton would have sewn up the nomination awhile ago, racking up a huge margin from wins in New York, New Jersey, California and Texas. Advertisement Conversely, had Republicans followed a propotional rule, the GOP race might still be going on, with Mike Huckabee, Mitt Romney and what would probably be a financially strapped John McCain fighting to gain ground in what by now would be a bitterly divided Republican party.