If Clinton picks Warren for V.P., is the U.S. ready for double-woman ticket?

When Hillary Clinton and Elizabeth Warren joined forces on the campaign trail this week and put on an energetic display of enthusiasm for each other, they did more than just fire up the crowd.

They also sparked this question: could the United States see an all-female ticket in this year's presidential election?

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Clinton is still the presumptive Democratic nominee — it won't be official until the party convention later this month — but there is plenty of speculation about whom she will pick as her running mate.

Warren, a Massachusetts senator who is considered a darling of the progressive wing of her party, is said to be on Clinton's list of potential vice-president picks.

Should Clinton pick Warren, or another woman, for her vice-president, it would be "a bold move," said Debbie Walsh, director of the Center for American Women and Politics at Rutgers University.

Warren only recently endorsed Clinton, to the great relief of her campaign, after staying neutral in the primaries. The two women are far from longtime pals but their relationship has had a breakthrough and Warren is acting as a highly effective surrogate.

The 67-year-old senator is winning praise from Democrats for the way she takes on Donald Trump, getting in Twitter spats with the presumptive Republican nominee and berating him during public speaking events.

In Ohio on Monday, Warren slammed Trump and gave voice to one of Clinton's campaign slogans by declaring, "I'm with her!" A satisfied Clinton stood off to the side listening to her new friend praise her brains, guts and good heart.

They clasped hands, raised their arms above their heads, waved to the cheering crowd and took note of a sign in the audience that read "Girl Power."

Was it a sign of things to come? Clinton is already set to make history by being the first female presidential nominee for a major party.

The woman card

Whether Americans are ready for a two-woman ticket is a question that's been posed to observers like Walsh in recent weeks.

"I think they are," she answered in an interview.

"I think Americans are ready to pull the lever for a woman president," said Walsh, and if it's another woman backing her up, it likely won't sway voters one way or the other.

"I'm guessing most people who would be reluctant to vote for two women might well be reluctant to vote for even one woman. So I think that's a bigger issue," said Walsh.

Clinton, in contrast to her 2008 presidential campaign, is embracing her gender this time around and responds to Trump's accusations that she is playing the woman card with the retort: "Deal me in."

When asked by David Muir of ABC News if she will deal another woman in and if the country is ready for a two-woman ticket, Clinton was non-committal. "I think at some point. Maybe this time, maybe in the future," she told Muir on the day she secured the nomination in early June.

Not ruling anyone out

In an interview with MSNBC's Rachel Maddow earlier in the year, Clinton was asked if she were the nominee would she have to discount a woman as her vice-president and choose a man in order to balance the ticket and appease those who might be uncomfortable with a two-woman ticket. The answer was no.

"I'm not ruling anybody out," Clinton said.

Kathleen Dolan, a professor at the University of Wisconsin-Milwaukee and author of When Does Gender Matter?, said Clinton is probably thinking about gender when considering her pick but that it's likely not her primary concern.

"She clearly has made different kinds of decisions in that she seems to be much more comfortable with the notion of her sex and the historic nature of her candidacy," said Dolan.

But if she does pick Warren, it's more likely going to be for her credentials, her appeal to progressive supporters of her rival, Bernie Sanders, and her approach to taking down Trump, and not because Clinton wants a double-woman ticket.

"Elizabeth Warren's greatest utility would be ... her attack-dog mentality — she has no concerns about getting right in there — and ... her progressive credentials," said Dolan.

And if she doesn't pick Warren, it's reasonable to assume it's not because Clinton is rejecting the idea of a two-woman ticket, but because she'd prefer someone else and for political calculations. If Warren were to step down from the Senate to be vice-president, for example, the Republican governor of Massachusetts would get to appoint a temporary replacement for her seat.

Democrats have a chance at regaining control of the Senate this fall and every seat will count. Giving one up is a risk many Democrats aren't crazy about taking.

Two women would shake things up

The pressure on Clinton when considering a woman running mate is coming from multiple directions.

"Some might want her to strike while the iron is hot," said Dolan, and capitalize on making history.

Some might want her to pick a man because of worries about the optics of two women, and Dolan said, "some voices might say 'Don't reject her because of a concern that the country would react to a two-woman ticket.'"

Americans are used to seeing same-gender tickets, just men, not women. Only twice have women been picked as running mates — Sarah Palin ran with John McCain in 2008 and Geraldine Ferraro with Walter Mondale in 1984.

Clinton has the opportunity to make history with her vice-presidential pick but if she doesn't, she's still changing the image of women in politics, said Walsh, of Rutgers.

"Hillary Clinton all by herself is disrupting our image of what a president looks like in this country and having two women, a presidential and vice-presidential candidate running together, [would really shake] things up," she said.