An international treaty promoting the rights of the disabled across the globe failed in the Senate on Tuesday, but it gave Sen. John F. Kerry, chairman of the Foreign Relations Committee, a chance to lay out a diplomatic vision that could serve as blueprint if he becomes President Obama’s next secretary of state.

The point of the treaty, Kerry said, was to urge the world to be more like the United States.

“This treaty isn’t about changing America. It’s a treaty to change the world to be more like America,” Kerry said.

The Massachusetts Democrat and former presidential nominee is considered a front-runner to succeed Secretary of State Hillary Rodham Clinton, who is expected to step down before the start of a second Obama term. Kerry, who was awarded the Silver Star for his service in Vietnam, has also been mentioned as a possible defense secretary, succeeding Leon E. Panetta.

The White House is expected to announce nominations to those positions soon, possibly as soon as this week, but the swirl of speculation has made it difficult for Kerry to navigate between his current job and his possible future.

The secretary of state nomination has already become contentious, as the other leading candidate, U.N. Ambassador Susan Rice, has come under attack from Republicans for misstatements made in the wake of the terrorist attack in Benghazi, Libya.

Kerry will not say whether he wants the job at State, but he has been boosted by many in the Senate who have attacked Rice and who say he would be a better choice.

Obama has been publicly supportive of Rice and has defended her against the GOP barrage. If he chose to nominate her, it would probably mean a contentious set of confirmation hearings in the Foreign Relations Committee, presided over by none other than John Kerry.

Republicans have signaled that they would welcome a Kerry nomination to any post with near unanimous approval, forcing the White House to think about how much of a fight they want to have over new Cabinet secretaries.

“If they were to nominate Senator Kerry for something, he would be pretty broadly applauded on our side,” Senate Minority Leader Mitch McConnell (R-Ky.) said Tuesday.

Many Democrats would prefer Kerry to be the nation’s top diplomat rather than Rice, according to several senators and aides who requested anonymity to speak freely about the president’s choice.

That is in part because the fallout from the Benghazi attack could turn a Rice nomination into the first tough vote of the 2014 election season. Others worry about expending political capital to secure a Rice confirmation during the heated talks to avert a fiscal combination of more than $500 billion in tax hikes and spending cuts set for January.

More important, however, senior Democrats believe Kerry has earned the nod through his dogged work since returning to the chamber after his narrow defeat to George W. Bush in the 2004 presidential election.

“He’d be a great secretary of state,” said former senator Ted Kaufman (D-Del.), who served on the Foreign Relations Committee.

Kerry, who declined to be interviewed for this article, has brushed off the speculation. Sen. John McCain (R-Ariz.), who worked with Kerry on the treaty for the disabled, introduced the Massachusetts Democrat as “Mr. Secretary” at a news conference Monday. Smiling, Kerry referred to the failed 2008 GOP presidential nominee as “Mr. President.”

There’s a certain irony to how Kerry has come to this moment as the preferred choice of many senators for secretary of state. Since first winning his seat in 1984, Kerry was viewed as aloof and distant, the sort of senator who spent most of his time angling for bigger things.

By 2000, he was on Al Gore’s shortlist for consideration as vice-presidential running mate, and by early 2003 Kerry practically abandoned the chamber to run full time for president. Returning after the 2004 loss, he continued to nurse White House ambitions until January 2007, when he declared he would devote his attention to the Senate and forgo another White House bid.

“It is the time to put my energy to work as part of the majority in the Senate, to do all I can to end this [Iraq] war and strengthen our security,” Kerry said then.

Still, after an early endorsement of Obama over Clinton in their epic 2008 primary, Kerry almost got the nod to become secretary of state, before the president adopted a “team of rivals” approach by selecting Clinton.

With Vice President Biden’s departure from the Senate, Kerry took over the Foreign Relations gavel as a consolation prize and impressed his colleagues.

Kaufman, Biden’s former aide who succeeded him in the Senate, credited Kerry for passing the “iron pants test” of a chairman: He sat through hour after hour of each hearing, questioning each witness, meticulously preparing.

Kerry has led efforts to forge deals on climate change and the “supercommittee” ’s bid for a grand bargain tackling the federal debt. He won approval for a nuclear arms treaty with Russia.

His big diplomatic moment came in the fall of 2009, when Afghan President Hamid Karzai was reeling from election results that kept him under the 50 percent threshold to avoid a runoff. Clinton dispatched Kerry to Kabul to persuade Karzai to call a runoff, and the Afghan leader relented only after Kerry delved into his own feelings of failure and despair about the 2004 outcome against Bush.

In recent years he has found new allies in Sen. Rob Portman (R-Ohio), his supercommittee counterpart, who is a sometimes partner on the 68-year-old senator’s still-aggressive daily bike rides. He has rekindled his friendship with McCain, his fellow Vietnam War veteran with whom he has a 25-year history of working and sparring.

Sen. Lindsey O. Graham (R-S.C.) rejected any suggestion that Republicans want Kerry to be nominated so that they could claim his Senate seat in a special election next spring, possibly by their departing colleague, Sen. Scott Brown (R-Mass.), who lost his reelection bid last month. “I just think that’s offensive to Senator Kerry,” he said.

Kerry’s rise in the Senate has come after the death of his friend, the late Sen. Edward M. Kennedy (D-Mass.), but many senators note that he has learned from his mentor — giving up on the presidential dream and digging into the Senate.

Now, because of his refocused devotion to the Senate, Kerry finds himself the favorite of his colleagues to finally leave the chamber for something bigger. But Kerry seems to have a new appreciation for the Senate as more than just a fallback position.

“What we do here in the United States Senate matters, not just to us but to people all across the globe,” Kerry said during Tuesday’s treaty debate. “And maybe some people here need to be reminded of that.”