Sen. John McCain's quest for the Republican presidential nomination was once seen as dead, but like those robots in the "Terminator" movies that reassemble themselves after being blown to smithereens, he came back. Five years ago, Sen. Barack Obama (Ill.) was a virtually unknown African American state senator. Now, he stands at victory's threshold ahead of tomorrow's Ohio and Texas Democratic presidential contests.

McCain's fortitude and Obama's charisma are beyond question. But each of these politicians also had a secret weapon that has received much less attention: For Obama, that would be the Democratic Party's decision to follow a system of proportional representation, where candidates got delegates in each primary in rough proportion to their overall vote tally in the state.

McCain (Ariz.), meanwhile, benefited from the Republican Party's decision to allow states to make their own decisions on how to allot delegates. In the largest states, the winner took all or most of the delegates.

To see why these rules played a decisive role in each man's success, imagine what would have happened if Obama had run under the Republican Party's rules while McCain ran under Democratic Party rules. Assuming Americans voted the same way they did, Sen. Hillary Rodham Clinton (D-N.Y.) would have assumed a commanding lead after the Super Tuesday primaries because she won most of the largest states. By contrast, the Republican nomination might still be wide open, because McCain failed to win many early-primary states decisively.

"There is so much focus on the horse race and whether candidates go negative and positive," said political scientist Jeffrey Karp. "What many people don't know is that the rules themselves play a role in determining what kinds of candidates win or lose. . . . If you had the proportional representation system on the Republican side, there is no doubt that race would have continued for quite a long time."

"Rules are not really transparent and do produce different outcomes -- that is something we know for a fact," added Karp, who studies the effects of various democratic setups in different countries. "Certain rules produce certain outcomes."

The Republican and Democratic primaries in California offer an especially useful window into how the race could have turned out differently if Obama and McCain had not had Article 13 and Rule 15 on their sides: Each man happened to win 42 percent of his party's vote in the state.

Obama picked up about two in five Democratic delegates, even though Hillary Clinton won 52 percent of the vote. McCain, meanwhile, won more than 90 percent of the Republican delegates in the Golden State, even though a majority of California Republicans did not vote for him. State GOP rules allot all the delegates in each congressional district to the candidate who wins the largest share of the vote in that district.

If the Democrats and Republicans had exchanged rule books, Clinton would have mopped up most of the delegates from California, New York and New Jersey -- populous states where she won majorities. Under proportional representation rules, McCain's 42 percent support in California would have given him fewer than half the 158 delegates he won in the Golden State; his 36 percent showing in Florida would have left him with barely a third of the 57 delegates he acquired there; and his 33 percent vote in Missouri would have left him with far fewer than the 58 delegates he picked up in that state.

Karp said one lesson from this is that although voters and journalists think intensely about the personalities and policies of the candidates, it is the party elders who write the rules of the game -- with little public input -- who often play a more powerful role than donors, the media -- even voters.

Karp, a Californian who works at the University of Exeter in England, said an analysis he conducted of 27 democracies around the world showed systematic differences between democracies with proportional representation and those that follow winner-take-all rules. As Obama's success highlights, proportional representation benefits candidates who do not start out with the wind at their back. It brings more voices into the political conversation, because minority views get a seat at the table. But in making the conversation inclusive, these systems sometimes prolong electoral uncertainty.

McCain's success, by contrast, illustrates what Karp found in democratic systems that follow winner-take-all rules: They quickly produce a winner. But by shutting out losers -- think Rep. Ron Paul (R), the Texas libertarian -- these rules undermine dark-horse candidates.

Studies show that across the world, liberals prefer proportional representation rules, while conservatives prefer winner-take-all.

Karp wryly noted that there is a third powerful group -- which sides with the conservative preference for winner-take-all. That would be the mass media, which love to have one candidate raise his or her fist in triumph after a primary in order to give the narrative of the race a sense of completion.

This is why the media have declared one clear victor at the end of every race this year, even when Democratic primaries and some Republican races have followed proportional representation rules -- and even when the majority of voters in the state rejected the "winner."