Kerry kept focus sharp amid drama

But behind the scenes, the House’s mid-December approval of a repeal of the military’s “don’t ask, don’t tell’’ policy threw Senator John F. Kerry’s efforts to win Senate approval of the New START treaty into sudden doubt.

Some Republicans were livid that the “don’t ask, don’t tell’’ repeal would be resurrected in the Senate in the waning days of a lame-duck session, and they threatened to walk away from the New START pact in retaliation.

The Massachusetts senator, chairman of the Foreign Relations Committee, privately warned the Senate leadership that months of negotiations and work were at risk of unraveling.

“It was a very delicate discussion,’’ Kerry said in an interview. “We certainly made them aware of the stakes without opposing anything. I am supportive of the ‘don’t ask, don’t tell’ repeal, and had called for years for the policy to be overturned. But my job was to get the treaty done.’’

Senate Majority Leader Harry Reid’s response: “Don’t ask, don’t tell’’ would remain on the table. The Senate would have to pass it, along with the treaty.

I know it makes it more difficult for you, Reid told Kerry, but this is the way it’s got to be.

In one of the biggest triumphs of his 25-year Senate career, Kerry managed to keep START on track over the next 10 days despite the fury of key Republicans. He and his allies, including Vice President Joe Biden and Secretary of State Hillary Rodham Clinton, fielded an intense lobbying campaign, headquartered in a quick-response war room that hummed with staff members.

The effort was aided by a critical last-minute, classified intelligence briefing and pressure from high-ranking military officers, applied at key moments throughout more than 70 hours of floor debate over the past week. In the end, yesterday’s bipartisan 71-26 vote in favor of the treaty completed a dramatic path for an arms pact that seemed close to failure several times in the past month.

“My hat’s off to the Democratic leadership; they’re running rings around us,’’ a frustrated Senator Lindsey Graham, a South Carolina Republican, told reporters this week as the last votes for START were falling into place. “They’re like Sherman going through Georgia here.’’

The victory helped Kerry ascend more firmly into the role of senior statesman from Massachusetts, a role he inherited after the death of Senator Edward M. Kennedy last year, as he spent hour after hour, day after day, alternately wooing and debating colleagues.

“He was masterful — diplomatic, cordial, patient, persistent,’’ said Joe Cirincione, president of the Ploughshares Fund, a foundation dedicated to peace and security. “He never let a single charge go unanswered. He accommodated where he could and resisted where he had to.’’

Kerry was a “workhorse’’ in the committee and on the Senate floor, said arms control specialist Michael Krepon, founder of the Stimson Center, a non-partisan arms control organization.

The treaty, which reduces strategic nuclear warheads on both sides by about a third, to 1,550, and sets up protocols for inspections, won the support of 13 Senate Republicans yesterday, including Scott Brown of Massachusetts.

In April, when President Obama and Russian President Dmitry Medvedev signed the pact, its prospects for ratification were questionable. Kerry, the administration’s point man on the treaty as chair of the Senate Foreign Relations Committee, started with just one Republican vote, that of Richard Lugar of Indiana.

Interviews with Kerry and other participants reveal how Kerry courted votes across the aisle and attempted to keep the partisan temperature of the debate as low as possible.

“He kept the debate above the partisan fray,’’ said Senator Jeanne Shaheen, a New Hampshire Democrat and member of Kerry’s committee.

Kerry allowed Lugar, a leading figure in halting the spread of nuclear weapons, to draft the resolution for ratification, the sort of gesture that is ignored outside the Capitol but which carries currency in the clubby world of the Senate.

“Kerry not insisting to write the resolution, and letting Lugar handle it was a significant decision,’’ said Jeffrey Lewis, a nonproliferation specialist at the Monterey Institute for International Studies. “That made it harder for Republicans to hurl stones.’’

At a critical turning point over the summer, Kerry faced an agonizing decision and chose a potentially risky path that paid off.

After his Foreign Relations Committee had held a dozen hearings on the treaty, supporters wanted Kerry to pass it out of committee before August congressional recess and get New START on the Senate calendar before the November elections.

But two Republicans on Kerry’s committee, Bob Corker of Tennessee and Johnny Isakson of Georgia, asked Kerry for more time. Kerry, seeking to avoid a strongly party-line committee vote, consented, gambling that a delay and broader support would be the better course.

“That delay meant that the treaty debate would be pushed into [the] lame-duck, but it also meant that Corker and others on the committee had the additional time they said they needed,’’ said Daryl Kimball, executive director of the non-partisan Arms Control Association, which promotes arms control policies.

The committee recommended the treaty in September by a 14-4 vote, with Corker and Isakson on board.

As the treaty came up for floor debate this month, Kerry set up a war room in the offices of the Foreign Relations Committee in the US Capitol building. Around a long conference table piled high with binders and briefing books, a dozen experts and staff members monitored the debate on laptop computers.

The war room staff delivered rapid responses to comments made on the floor, digging up facts that were fed to Kerry by e-mail, in person or on paper walked to the Senate floor.

Reid had planned to file a motion on Saturday to cut off debate on the treaty and proceed to a final vote. Kerry called him early that morning and asked Reid to wait a few days, to allow anger over the “don’t ask’’ vote to fade. He said he did not want to risk falling short and having the treaty debate bounce into the new Congress in 2011.

“I saw political difficulty, damage, and who knows what the mood is going to be next year?’’ Kerry said. “We pushed forward despite the fact, frankly, that we didn’t have the votes.’’

A key swing vote was Brown, Kerry’s colleague from Massachusetts, whom Kerry courted over the telephone, on the Senate floor, and in the halls. He would prove to be pivotal on Monday, when the Senate met in closed-door session to discuss classified aspects of the treaty.

Kerry and the administration also brought in two high-powered experts — a four-star Air Force general, Kevin P. Chilton, commander of the US Strategic Command, and Tom D’Agostino, head of the National Nuclear Security Administration, the agency that manages the nation’s nuclear stockpiles — to answer questions from wavering senators, including Brown.

Shortly after that meeting, Brown unexpectedly walked into a crowd of reporters in a hallway outside the Senate chamber and announced that he had performed his “due diligence’’ and would support the treaty. Brown, through his office, declined to be interviewed for this story.

“He took his time, asked a bunch of questions,’’ Kerry said of Brown. “And I credit him with that. He wanted to be satisfied.’’

Bryan Bender of the Globe staff contributed to this report.