Sen. Elizabeth Warren looked very much like a running mate as she stood alongside Hillary Clinton and electrified a large crowd in battleground Ohio this week, punching the air, eviscerating Republican Donald Trump and declaring: “I’m with her. Yes, her.”

Not everyone was impressed — including some high-profile supporters of Democratic runner-up Bernie Sanders, who remain hostile to Clinton and bitter that Warren (D-Mass.) didn’t back the senator from Vermont during the primaries.

“If Clinton thinks that picking Elizabeth Warren is going to sweep up all the enthusiasm for Bernie, I just don’t see it,” said Burt Cohen, a former New Hampshire state senator who served on Sanders’s steering committee in the state. “It’s about issues, not about personalities.”

Such sentiments challenge one assumption about Warren as vice presidential nominee: that she would validate Clinton’s progressive bona fides and help bring along disaffected Sanders voters who still view the former secretary of state warily.

There are other arguments against Warren, according to allies and advisers from both camps: concern that the country is not ready to support a two-woman ticket; Clinton’s own tendency to avoid risk; her desire to choose a governing partner, not a surrogate attack dog; and the reality that the two women simply do not know each other that well.

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There is also the fact that Warren’s interim successor in the Senate would be named by a Republican governor, complicating Democratic efforts to take control of the chamber and help push Clinton’s agenda. And her value is a double-edged sword; even if she does prove effective in rallying the party’s left wing, her star power could show up Clinton on the campaign trail — and showcase Clinton’s own struggle to energize these crucial voters.

Warren has made no secret of wanting the job, telling MSNBC host Rachel Maddow weeks ago that she considers herself fully able to do it. But even some of her own boosters, who would like to see her on the ticket, are skeptical there is a sufficient well of trust for Clinton to pick her.

The two women have had two private meetings that intimates described as remarkably warm and productive. Their most recent, initiated by Clinton ahead of their joint campaign appearance Monday in Cincinnati, included a ride together from Clinton’s hotel to the event site.

Boosters of the potential ticket, including Mary Anne Marsh, a Democratic consultant based in Boston, argue that adding Warren would “double down” on Clinton’s appeal to women — a more important segment of the electorate than ever this year — and add an already proven voice for “going after” Trump.

Marsh argued that even if Sanders backers are not thrilled about a Clinton-Warren ticket now, they would support it in the fall.

“How can you call yourself a progressive if you can’t support a ticket of Hillary Clinton and Elizabeth Warren over Donald Trump and fill-in-the-blank?” Marsh asked. “That choice is pretty clear.”

Sen. Elizabeth Warren (D-Mass.) is on Democratic presidential candidate Hillary Clinton's short list of potential vice presidential candidates. Here's what you need to know about her. (Sarah Parnass,Osman Malik/The Washington Post)

Warren, by far the most visible of Clinton’s list of potential running mates, rose to prominence for her sharp criticism of Wall Street’s excesses and fierce advocacy on behalf of the nation’s consumers.

Other potential picks are also being vetted by the Clinton team, including Sen. Timothy M. Kaine of Virginia, viewed as a safer selection, and Housing and Urban Development Secretary Julián Castro, who could energize the Hispanic electorate.

At the Cincinnati rally, Warren called Trump “a small, insecure money grubber” and “a thin-skinned bully who is driven by greed and hate.” She said he was only interested in making America “great again” for “rich guys, just like Donald Trump.”

She has also engaged Trump in a war of words in one of his favorite arenas — Twitter — and not pulled any punches in assessing his candidacy or character. “There’s more enthusiasm for @realDonaldTrump among leaders of the KKK than leaders of the political party he now controls,” Warren tweeted in May, for instance.

Although Warren’s role as a darling of the left was in some ways eclipsed by Sanders’s rise on the presidential campaign trail, she has re-emerged since Clinton assumed the mantle of presumptive nominee — and Sanders’s profile has done the opposite.

In fact, notwithstanding the many Sanders supporters who say they would never vote for Clinton — or who criticize Warren for joining forces with her — recent polling suggests that most Democrats have actually lined up behind the presumed nominee.

A Washington Post-ABC News poll released last week showed that only 8 percent of Sanders supporters said they would back Trump over Clinton in the general election, down from 20 percent a month earlier. The greater fear for Democrats is that Sanders voters might simply stay home.

Leading Sanders supporters say they are focused on the more immediate term, where the chief aim is getting Clinton and the Democratic Party to embrace many of the ideas that Sanders built his campaign around.

While they have had some success moving the party’s platform committee in that direction, other Sanders priorities — such as universal health care and reining in the nation’s trade policies — remain elusive.

They also express frustration that the Clinton team has not treated Sanders with the respect they say he earned during primary season, when he won 22 states and 13 million votes. They say putting Warren on the ticket alone will not fix that.

“I know that people think he’s easily interchangeable with another progressive, but he’s not,” said Nina Turner, a former Ohio state senator and frequent Sanders surrogate. “It is a mistake to think she could just step in.”

Talks in recent weeks between the Clinton and Sanders campaigns have been largely focused on policy issues, and the vice-presidential slot has not been a topic of conversation, according to a Democrat familiar with the discussions. In television interviews, Sanders has said he would like to see Clinton pick a progressive. When asked, he has complimented Warren. But Sanders is not publicly advocating for her.

There remains the question of what Warren would bring to the ticket — and what she would not.

RoseAnn DeMoro, executive director of National Nurses United, the first nationwide labor union to endorse Sanders. said she has heard “zero interest” from fellow Sanders supporters of late about seeing Warren on the ticket.

“I’m just not hearing it,” DeMoro said, adding that many Sanders backers were disappointed that Warren endorsed Clinton last month as the primaries wound down.

“People think because she’s with Clinton, she’s gone over to the dark side,” DeMoro said of Warren.

The Young Turks, an online news network popular with progressives, has held heated debates on whether Warren threw away her credibility by backing Clinton. On a segment about her official endorsement, the network’s major hosts ranged from minor disappointment to rage over Warren’s refusal to back Sanders.

“If you’re a Democrat, it’s super-easy to throw a punch against Donald Trump,” said host Cenk Uygur, who frequently introduced Sanders at California rallies. “But it was important to throw a punch against the powerful in your own party and stand up for progressive values.”

Jimmy Dore, another host, argued that Warren lost credibility with progressives when she did not defend Sanders against Clinton’s attacks in the primaries.

“You saw what they were doing to him,” Dore said. “You saw them calling him misogynistic. The least you could have done is said: All of that is crap! How dare the liberals and people who call themselves progressives treat the one true liberal in this race like that. That would have been really helpful.”

Neil Sroka, spokesman for Democracy for America, one of the first groups to endorse Sanders, said such sentiments are not emblematic of where most Sanders voters are.

“Frankly, I think it’s a relatively small number of loud voices online who are saying negative things in the wake of [Warren’s] endorsement,” said Sroka, whose group was a member of the effort launched in 2014 to draft Warren to run for president.

“The discussion’s a lot more nuanced than that,” Sroka said. “Sure, 98 percent love Warren and would support her no matter what she did, unless it was working for Goldman Sachs or something. There’s also an appreciation for the difficult stance she took. She was the only female Democrat in the Senate who didn’t endorse Clinton. That was a really bold, difficult stance. If anyone’s ever been in a work environment where everyone around you took one side, you know how difficult that must have been.”

Erin Bilbray, a Sanders supporter and member of the Democratic National Committee from Nevada, said she has heard from some “disgruntled” Sanders supporters “whose wounds are still fresh at this point.”

But Bilbray said adding Warren to Clinton’s ticket would be “terrific.”

“Her policies are the kind that brought me to the Sanders campaign in the first place,” Bilbray said.

Mike Carberry, a county supervisor in Johnson, Iowa, who backed Sanders, said for some supporters of the senator from Vermont, there is little Clinton can do to bring them on board.

“He brought a lot of people into his campaign who are still having a hard time accepting that Bernie may not be the nominee,” said Carberry, who was among those who initially urged Warren to run for president. “The seasoned people who’ve been through this before will get behind Clinton, regardless, because they want a Democratic president. Some of the new converts to politics will take more time to come around.”

On that count, adding Warren to the ticket “couldn’t hurt,” Carberry said.

Karen Tumulty contributed to this report.