Donna Brazile, the former interim head of the Democratic National Committee, says in a new book that she considered replacing Hillary Clinton as the party’s 2016 nominee with Vice President Joseph R. Biden Jr. after Mrs. Clinton suffered a fainting spell, according to an account published Saturday by The Washington Post.

In her book, Ms. Brazile writes of her frequent frustrations with the Clinton campaign, saying that she would tell its leaders that she could invoke powers from the D.N.C. charter to oversee the process of replacing a nominee if she were to become disabled, something that has never happened in the modern era of presidential elections.

The Post quotes Ms. Brazile as writing, “Again and again I thought about Joe Biden,” adding that “no matter my doubts and my fears about the election and Hillary as a candidate, I could not make good on that threat to replace her.” The Post reported that Ms. Brazile had considered replacing Mrs. Clinton after she briefly collapsed in New York City following a Sept. 11 commemoration ceremony.

But the rules do not give that power solely to the chairwoman of the party. That decision is ultimately made by the D.N.C. after consultation with Democratic leaders in Congress and Democratic governors.