WASHINGTON » John Kerry has made no secret of his ambitions as secretary of state. On a visit to the Church of the Nativity in Bethlehem last March, he watched as two bearded patriarchs pleaded with President Barack Obama to bring peace to the Holy Land. Clasping their hands, Kerry said, "I’m going to work as hard as I can to get it done."

Eight months later, Kerry’s effort to broker peace between Israel and the Palestinians is still an uphill struggle. But he may be poised to begin delivering another major goal Obama has long sought: an agreement with Iran to curb its nuclear program.

If the United States and its five negotiating partners come within striking distance of an interim agreement with Iran, Kerry is likely to fly to Geneva at the end of the week to try to seal the deal. It would be a rare win for a White House that has been reeling from the botched rollout of the health care law, a stalled legislative agenda and doubts about Obama’s credibility.

It would also ratify Kerry’s status as the biggest surprise of the president’s second-term Cabinet: a hyperactive diplomat who plunges into seemingly intractable problems, improvises furiously along the way — making gaffes from time to time, but occasionally devising solutions that have helped Obama out of messy situations like the impasse over a security agreement with Afghanistan.

None of this is to say that Kerry and Obama are personally close. With his patrician style and oratorical flourishes, Kerry is not a natural match for the president. Even now, his long-winded explanations — a legacy of his 28 years in the Senate — sometimes test Obama’s patience, although Kerry’s aides say he has "learned to speak the president’s language."

But Kerry, current and former administration officials say, has won growing respect from Obama, who has tended to keep his Cabinet secretaries at arm’s length and on a short leash. As the Iran talks heated up in recent weeks, officials said, the White House deferred to Kerry’s argument that he should thrust himself into the negotiations to try to bridge differences between Iran and the West.

"Kerry made the judgment about whether he could move the ball down the field," said Benjamin J. Rhodes, a deputy national security adviser. "The president supported Kerry’s judgment."

Once in Geneva, Kerry confronted resistance from French diplomats, who worried that the deal taking shape with Iran was too lenient. Conducting much of the negotiation in French, Kerry incorporated their points into the broader proposal, which still fell short of an agreement with the Iranians by what he later said were four or five phrases.

Kerry’s prodigious energy and desire to make a mark have made him a more activist secretary of state than his predecessor, Hillary Rodham Clinton, and so far at least, more willing to take risks than Clinton, who may have another presidential campaign in her future. Aides say Obama has marveled at how Kerry spent seven hours with Prime Minister Benjamin Netanyahu of Israel, negotiating the fine points of a peace deal with the Palestinians.

David Axelrod, an adviser to Obama, watched the two men in close quarters when Kerry played the part of Mitt Romney during the president’s debate prep sessions – and Kerry infuriated the president with his needling imitation of the Republican candidate.

"They may seem different, and they are different," he said, "but they’ve cooperated closely and become friendly."

Some of their differences, however, have played out in public. When Obama reversed course and decided not to order a military strike on Syria in August to punish it for a deadly chemical weapons attack, he did not bother to tell Kerry of his decision until after he had briefed his aides, even though Kerry had become the most public advocate of military action.

If Kerry was offended, he did not show it, throwing himself into the effort to flesh out and win support for a Russian-inspired plan to eliminate Syria’s chemical weapons stocks — a plan he had helped set in motion with a seemingly offhand remark at a news conference.

"He helped bail the president out of a no-win situation, in which he might have had to use force, with a divided country and Congress," said Robert Danin, a former State Department official who has worked on the Middle East peace process. "It’s not Nixon and Kissinger, but Kerry’s actions have made him relevant to the White House."

Obama’s aides also praise Kerry’s efforts to negotiate the postwar security deal with President Hamid Karzai of Afghanistan — an effort that required marathon talks between the two men but which Kerry said Wednesday was finally completed.

Still, Kerry’s overall record at the State Department is mixed: His Syria diplomacy, while face-saving for Obama, has left the United States woefully short of its stated goal of persuading President Bashar Assad to yield power.

To some analysts, his focus on a peace accord between the Israelis and Palestinians is quixotic, given the poor climate for a deal. Among leaders in the region, the perception is that this is very much his project: a diplomatic effort the White House will fully embrace if it succeeds but that can be laid at Kerry’s doorstep if it fails.

Kerry’s indefatigable commitment is the biggest reason Obama listed Middle East peace as one of his top priorities in the region when he spoke at the United Nations in September. But the policy review that led to that speech also underscores the limits of Kerry’s role.

The review was conducted by Susan E. Rice, the national security adviser, with a small circle of White House aides. Although Rice briefed Kerry over weekly lunches, neither he nor any member of his staff took part in the deliberations.

There are signs that Kerry’s galloping style is causing friction with Rice. On a recent visit to Egypt, he emphasized continuity with Egypt’s generals, despite their ouster of President Mohammed Morsi last summer. Rice was angry, officials said, that Kerry did not put more pressure on the generals for their brutal crackdown.

"He’s pushing the mandate of his authority," said Danin, now at the Council on Foreign Relations. "We have yet to see if this White House will stand by him, should he fail to produce results."

A major test will come at the Iran talks, which began Wednesday. Kerry has not been as deeply immersed in them as in Syria or the peace process. But when U.S. negotiators flew to Geneva this month, he began thinking about how he could make a difference. On Nov. 7, in the middle of a nine-day tour of the Middle East, he decided to take the plunge.

The move was classic Kerry. He went to Geneva not calculating that an agreement was assured, but with the hope that his long ties with foreign ministers and negotiating stamina would at least enable him to narrow the differences with the Iranians.

But Kerry’s improvisational approach posed challenges. With other foreign ministers racing to catch up with Kerry, attempts to synchronize the positions of the major powers were carried out on the fly. That proved to be a problem for the French, who had long taken a tough stance with Iran and who had their own demands.

Warning that too hasty an agreement would be a "sucker’s bet," the French foreign minister, Laurent Fabius, insisted on placing stricter curbs on a heavy-water reactor Iran is building near Arak. Kerry incorporated the French proposals, and administration officials insisted that reports of disunity among the allies were overblown.

Still, the backstage drama prompted Iran’s foreign minister, Mohammad Javad Zarif, to complain on Twitter that the West was moving the goal posts. It also fed anxieties on the part of Israel and in Congress that the White House was in too much of a rush to get an accord.

For some longtime friends, Kerry’s headlong approach to diplomacy illustrates a larger truth about him. As Sen. John McCain, R-Ariz., who shares with Kerry both a record of distinguished service in Vietnam and a failed run for the White House, put it, "I believe that for John Kerry the reality is often as he wishes rather than as it actually is."