The woman in line to become the face of America abroad has been hit with strong opposition from Republicans, but her many defenders see a principled, engaged and highly intelligent force

It is a remark that has come back to haunt Susan Rice.

At the height of the 2008 election cycle, the then foreign policy adviser to Democratic candidate Barack Obama took a swipe at Republican opponent John McCain who had once donned body armor to visit a Baghdad market on a visit to Iraq.

Rice, who is now battling for a nomination to be America's next secretary of state, did not hide her scorn at McCain as she spoke about Obama's upcoming trip to the Middle East: "I don't think he'll be strolling around the market in a flak jacket," she said.

It was the sort of blunt, insulting language common during a hotly contested election and also fairly common with Rice. But it is hardly the sort of sentiment suited for the position of America's top diplomat. Now, McCain is making Rice pay.

The Arizona senator is leading a fierce charge against Rice, using her public comments over who was behind a September terrorist attack in Libya that killed a US ambassador to beat back her hopes of replacing Hillary Clinton.

While the attack on the Benghazi consulate is the nominal reason for McCain vowing to "do everything" to block Rice, there are some who see personal motivations rooted in an aspect of Rice's personality that has reared its head repeatedly throughout her career: her bluntness.

"The fact that Susan ridiculed him for wearing body armor walking in a market really stuck in McCain's craw. That made it very personal. He got called a sissy by a woman who has never served in the military," said one close Washington observer of the spat.

McCain and the Republicans are not alone in picking up on Rice's temperament when it comes to being the next face of America abroad. Influential Washington Post columnist Dana Milbank wrote a scathing piece in which he concluded Obama needed someone with more sensitivity. "Obama can do better at State than Susan Rice," he wrote. The Russians too have been grumbling with anonymous leaks declaring Rice "too ambitious and aggressive".

Yet Obama has doubled down on Rice. He warned, in a rare western gunslinger kind of way, that if McCain and others "… want to go after somebody, they should go after me". Obama has put his presidential authority on the line to get his way.

That might be because Rice is far more than just someone known to speak bluntly. At her current job as US ambassador to the United Nations she is seen as a savvy and tough operator who has done much to repair America's status there after the willful neglect of the George W Bush years. She has had a glittering career in foreign affairs via senior roles on the National Security Council, being America's top diplomat in Africa and numerous engagements in wonkish think-tank circles.

Her many defenders see her as principled, engaged and highly intelligent. "She is a sharp woman. She's Type A. She likes to achieve her goals and she is tireless," said Michael Williams, co-editor of Power in World Politics and who worked with Rice on the 2008 Obama campaign.

Indeed Rice has been a high-flying achiever since high school, racking up an enviable CV in academia and practical diplomacy all over the world. But, in the vipers' nest of Washington DC, she has now made enemies. Nor do they seem likely to be placated. Rice earlier this week held private meetings with McCain and other Republican senators, but some came out even more critical afterwards, including moderate Maine senator Susan Collins. Again, a few observers see Rice's perceived prickly personality at work. "Maybe she rubs them up the wrong way when she's trying for a love in," speculated one.

No 'sharp elbows'

But what sort of woman is Rice? She does not have a hard luck origin story. Unlike her President, there is no tale of a struggling single mom. Nor, like her famous Republican namesake Condoleezza Rice, did she grow up in a racially divided South. Instead Rice is a child of privilege and wealth among the Washington DC establishment. Rice's father, Emmett J. Rice, was a governor of the Federal Reserve, and her mother, Lois Dickson Rice, is an education expert at the famed Brookings Institution – an august body Rice herself would eventually join.

Rice, 48, attended the city's famous National Cathedral School, where generations of Washington's elite offspring have been educated. She stood out in that heady crowd, becoming president of the student council and class valedictorian. She also showed a tough streak being known for her physical play on the basketball court (something appreciated by the hoops-loving Obama) and earning the nickname "Spo" which was short for "Sportin".

She left an impression. After Milbank penned his critical piece one of Rice's former teachers, John Wood wrote in to fiercely defend his ex-pupil.

"I saw no 'sharp elbows' whatsoever. In a very demanding school she was laid back and funny, but also focused and hard-working," Wood wrote, pointing out the school still used the revised honor code that Rice drew up three decades earlier.

Rice has often been good at getting people to step up for her. The most important being Bill Clinton's secretary of state, Madeline Albright, who has long acted as a mentor. The Rice and Albright families knew each other socially as Rice grew up. Albright helped her get her first job at State in 1997, when she catapulted in as head of the Africa desk to the reported annoyance of older Africa hands and the African press. When she was sworn in, as one of the youngest assistant secretaries of state ever, Albright was reported to have told Rice's mother: "I feel like I'm swearing in family."

But Rice shone in the role which saw her deal with major crises like the bombing of the US embassies in Kenya and Dar es Salaam and bloody conflicts in Congo, Eritrea and Ethiopia. "She was a wonderful ambassador for Africa policy. She knows her dossiers; she is hard and she is sharp," said Professor William Zartman, an Africa expert at John Hopkins University who worked with her on African policy.

Indeed Africa has long been Rice's expertise. The transition from white rule to black rule in Zimbabwe was the subject of her prize-winning doctoral thesis. She worked for five years for Bill Clinton at the National Security Council, specialising in peacekeeping and Africa and increasingly working as an advocate for a more interventionist policy. That belief sprang out of a failure to prevent the 1994 Rwandan genocide and was cemented on a visit to the benighted country where Rice saw piles of bodies in the streets.

She told one reporter of the experience: "I swore to myself that if I ever faced such a crisis again, I would come down on the side of dramatic action, going down in flames if that was required." That explains, many say, much of Rice's tough-minded stances on issues like the intervention in Libya, taking a hardline on Sudan and her open criticism of China and Russia when they blocked a UN resolution condemning brutality in Syria (she called it "disgusting").

"The part that people miss in all of this is that she is a believer. She really does care," said John Norris, a director at the Center for American Progress.

That passion is coupled with a ferocious work ethic. While pregnant with her first child Rice did not take any time off until the day before the birth. It also comes with a keen intellect and a breadth of knowledge that few match. While working with Rice when she was a foreign policy adviser to Obama, Williams remembers her briefing foreign ambassadors at the Denver Democratic convention in 2008. He was stunned at her mastery of issues far outside what he had understood to be her specialty. "I had not expected her to be so clued in," Williams said.

But that work ethic – unlike the famously single Condoleezza Rice – has not prevented her from a suburban family life. Rice met her husband, Canadian television journalist Ian Cameron, while at college at Stanford. The pair have two children and live in Washington with Rice commuting to New York for UN duties. That has sparked a few complaints that she has missed important meetings, but most analysts see Rice as having done well at the UN – the brusque treatment of Russia and China aside.

"At the UN she has put a bunch of savvy political professionals in there who know the UN pretty well," said Norris.

She has also developed a good working relationship with Hillary Clinton, which was not always guaranteed. When Rice – who served so long in the Clinton White House – plumped for Obama in 2008 it was seen by some around Hillary as a betrayal. But, it seems, the wounds have healed. Nor is that the first time.

'She is a sharpie. But she follows orders'

For all those who say she has a reputation as a bare-knuckle fighter, there are others who say it is just the nature of her work and professional passions.

Michael O'Hanlon, a fellow at Brookings, clashed repeatedly with Rice when he worked for Clinton's 2008 campaign. "We battled hard yet parted as close friends despite it all. She is very likeable and appealing," O'Hanlon said.

Indeed some believe the current sniping around Rice has a lot to do with sexism. Few male political figures are critiqued for their aggression. There is also serious politics at play. Almost 100 Republican members of the House of Representatives have come out against Rice now, but that might be simple party strategy. If Rice fails to make it through the nomination process, then the next choice is likely to be Massachusetts senator John Kerry, which will open up his Senate seat for a fresh election.

But that sort of cynical politics is the world Rice has lived in for a long time. Nor has Obama exactly carried out a US foreign policy in his first term defined by morality, rather than Realpolitik. Obama surged troops in Afghanistan, escalated drone attacks all over the globe and failed to close Guantánamo Bay.

"Barack Obama became a less ideological but more effective version of George W Bush," said Professor Aaron Miller, a vice-president at the Woodrow Wilson Centre.

That is a philosophy that almost no one thinks Rice will change. Clinton has her own vast power base and has been seen as an effective global ambassador for America, yet her fundamental foreign policy is seen as still emanating from the White House. But Rice is far closer to Obama than Clinton and thus far less likely to be able to push back. Her recent career has been tied closely to Obama's and she is a member of his inner circle.

"She is tough. She is a sharpie. But she follows orders," said Zartman.

That last characteristic – one rarely mentioned in most profiles – may end up being the most important personality trait of all if Rice does indeed ascend to America's diplomatic hot seat.

"Will American foreign policy change? That is not a question up to Rice. That is a question driven by Barack Obama," said Miller.