The practice of giving extra weight to the most partisan states has its roots in the bitter Republican Party divisions of a century ago, said Barbara Norrander, a professor at the University of Arizona who wrote “The Imperfect Primary: Oddities, Biases and Strengths of U.S. Presidential Nominating Politics.”

After William Howard Taft won the 1912 Republican nomination over Theodore Roosevelt — in part because of the support he got from delegates from the South, which was then heavily Democratic — some Republicans questioned the wisdom of giving a Democratic region so much sway when it comes to deciding whom the Republicans should nominate, Professor Norrander said. Four years later, she said, the Republican Party devised a system that took delegates away from Southern states, and not long after that the party moved to adopt a system of awarding bonus delegates.

Members of both parties said that it made sense to give more sway to party loyalists.

“The theory behind it is plain enough: to reward success,” said David A. Norcross, the former chairman of the rules committee of the Republican National Committee.

Mr. Norcross noted that as parties have turned to primaries and caucuses in recent decades, the importance of conventions and delegate counts has waned, lessening the impact of bonus delegates. Most candidates now win the nomination by winning enough of the early contests to drive their competitors from the field. “Momentum usually takes the place of delegate counts,” he said.

“If you got down to the point where you’re counting, like we were in ‘76, then it would matter,” he said, referring to the 1976 Republican National Convention, when President Gerald R. Ford and Ronald Reagan arrived without the number of votes needed for the nomination, but Mr. Ford swayed enough delegates to win narrowly on the first ballot. “My prediction is we’re never going to get there.”

Elaine C. Kamarck, a lecturer at the John F. Kennedy School of Government at Harvard who has been on the Democratic National Committee’s rules and bylaws committee and has served as a superdelegate, said that while the extra weight given to the most partisan states has had little impact in past elections, it could play a role in a tight race.

“Once you are in a delegate race, once you don’t have an early knockout, then everything matters,” said Ms. Kamarck, the author of “Primary Politics: How Presidential Candidates Have Shaped the Modern Nominating System.” “The allocation of delegates matters, the distribution of the delegates across the states matter, who’s going to win more Congressional districts matters.”