And should Mrs. Clinton become the nominee, she would no doubt seek to balance her ticket with various considerations to increase her chances, even on the margins, of victory, including regional and gender diversity.

Senator John McCain and Vice President Walter F. Mondale opted for gender diversity with Gov. Sarah Palin of Alaska and Representative Geraldine A. Ferraro of New York as running mates; Mrs. Clinton might also do that to win back the support of white men, whose backing of President Obama fell from 41 percent in 2008 to 35 percent in 2012, roughly as low as in the era of Richard M. Nixon.

But a generation of presidential races has demonstrated the flaws in those calculations. Mitt Romney, the former governor of Massachusetts, was not well served by the Midwestern roots of his running mate, Representative Paul D. Ryan; the two handily lost Mr. Ryan’s home state, Wisconsin. Female running mates did not lift Mr. Mondale or Mr. McCain. In contrast, Al Gore and Bill Clinton made up for their regional hegemony with a joint veneer of youthful change agency.

While there is no precedent for two women running together for the highest office in the United States on the ticket of a major party, women have made gains in groups.

In 1992, when two Senate seats became open in California, the political conventional wisdom concluded that the two Democrats running — Ms. Feinstein and Barbara Boxer — would not possibly win in the same cycle. They are both serving today and lead powerful Senate committees.

New Hampshire now has two female senators — Jeanne Shaheen, a Democrat, and Kelly Ayotte, a Republican — and its entire top elected slate in the state is made up of women.

And it is now common to see women at the upper reaches of power in Washington. “It was once hard to imagine a female secretary of state,” Ms. Mandel said. “Then we had three in a row.”