WASHINGTON- If US Secretary of State John Kerry pulls off a nuclear deal with Iran, it will be a singular achievement in a long career in which the grand prize has eluded him.His 2004 presidential election loss, lack of legislative monuments despite 28 years in the Senate, and failure, like many before, to bring peace to Israelis and Palestinians have contributed to a view that he struggles to seal major successes.The 71-year-old has expended remarkable energy in pursuit of what would be an historic agreement with Iran, flying tens of thousands of miles and holding dozens of meetings with his Iranian counterpart. He plans to attend the possible last stage of the nuclear talks in Vienna ahead of a June 30 deadline, despite breaking his leg late last month.Kerry's negotiating style with Iran is a cocktail of boundless energy, tactical flexibility and occasional hardball, according to officials involved in the talks.Still, critics say he has broken some classic rules of negotiation, overshadowing the principal US negotiator Wendy Sherman; meeting Iranians regularly rather than holding himself in reserve; and exuding an air of eagerness for a deal.Supporters say no one will work harder for an accord and that his unsuccessful efforts to end Syria's civil war or the Israeli-Palestinian conflict shows he is willing to risk failure while tackling the hardest problems.DIPLOMACY BY ODOMETER? Kerry's determination to headline the talks is obvious from his travel schedule.In the last six months, he has met his Iranian counterpart, Mohammad Jawad Zarif everywhere from Geneva, Paris, Davos, Lausanne, Montreux and Munich to New York City.Veteran negotiators say Kerry's willingness to meet the Iranians so often makes it look like he is chasing a deal, rather than intervening only when the toughest issues remain."Being secretary of state by odometer is a very false measure," said Richard Armitage, a deputy secretary of state under Republican President George W. Bush."The administration is too eager," added Armitage. "Iranians can smell this. And if the secretary of state is willing to airmail himself in at the 11th and a half hour every time, what incentive is it for the Iranians to give you their bottom line?"Armitage reserved judgment on the nuclear deal, stressing the need for short-notice inspections of Iranian facilities to ensure Tehran is not cheating.Under a deal, Iran would curb its nuclear work in return for sanctions relief. Crucial differences remain between six major powers and Iran, even after a framework deal was agreed on April 2.Senior US officials disputed that Kerry has undermined Sherman, saying the two work extremely well together and that some issues can only be resolved at the highest levels. They also said Kerry had shown himself willing to play hardball.With a deadline looming in Lausanne on March 27, Kerry paid a late-night visit to Zarif's hotel room to say that if the Iranians didn't show flexibility, he would tell foreign ministers flying in to finalize the interim accord not to turn up, the officials said.The next morning, Iran raised an issue it previously refused to negotiate, the officials said, declining to identify it. An Iranian official declined comment.AT THE TABLEKerry became secretary of state in 2013 after a long Senate career in which he was not known as a deal-broker.He was credited with helping normalize US relations with Vietnam and investigating the Bank of Credit and Commerce International. He poured his heart into long-shot bipartisan climate change legislation that failed in 2010.Some who watched Kerry up close describe a senator eager to take on complex, sometimes arcane issues and doggedly pursue them even when it became clear to others they were lost causes."There's a sense that he loves a mission that might lead to glory," said a person with knowledge of Kerry's work on an ill-fated "super committee" formed to find $1.5 trillion in government savings. "He's like a shark; he can't stop swimming."The day after the committee collapsed, Kerry summoned some members to his office in search of a breakthrough."Everybody but him knew it was a bridge too far," this person said.In the final push for the April framework deal, Kerry left the negotiating room to take a call from the White House and returned with a markedly tougher stance, said several officials briefed on the incident.These officials interpreted it as a sign of White House control. A senior US official said he was unaware of the White House telling Kerry to harden his stance and that if such an incident occurred, it was likely a ploy to pressure Iran."Since it was not always his modus operandi, when he did show a very blunt edge, it got the point across very effectively," said a former senior official for one of the six powers negotiating with Iran.Critics and supporters of Kerry agree that an Iran deal would seal Kerry's legacy despite his earlier failures."If Kerry hits one grand slam no one will remember his other strikeouts," said Karim Sadjadpour, a Carnegie Endowment for International Peace analyst.