Corker said Hagel's affiliation with a group called Global Zero is "just concerning." The group is devoted to eliminating nuclear weapons, and Corker doesn't want the U.S. nuclear arsenal to be weakened.
But Kerry said Hagel is "realistic." And the Global Zero initiative "is not talking about today's world."
"It's a goal. It's an aspiration. And we should always be aspirational," he said. "But it's not something that could happen in today's world, and nor could any leader today sit here or in any other chair and promote to you the notion that we ought to be cutting down our deterrent level below an adequate level to maintain deterrence."
He said Hagel is "a strong patriotic former senator, and he will be a strong secretary of defense."
Kerry touched on various other issues, including Latin America, Mali, Somalia, Russia, the plight of women and girls in Asia, and the foreign affairs budget being "less than 1% of the entire budget of government."
Foreign policy, he said, is shaped and defined by many factors.
They include "food security and energy security, humanitarian assistance, the fighting against disease and the push for development," he said.
He noted the importance of investment, and cited the competitive challenge of China's involvement in Africa.
"China is all over Africa -- I mean, all over Africa. And they're buying up long-term contracts on minerals, on -- I mean, you name it. And there're some places where we're not in the game, folks. And I hate to say it. And we got to get in. But it takes a little bit of resourcing. Believe me, somebody's paying for those folks to be over there. And somebody's investing in their investment of time," Kerry said.
"We have to be prepared, because I think that what we bring to the table is, frankly, a lot more attractive than what a lot of other countries bring to the table. People like to do business with American businesses. People -- we're open, we're accountable. We have freedom of creativity and other kinds of things. And I think that if we can organize ourselves more effectively in this sector we can win."
He also talked about the importance of getting into the clean energy market, calling it a job creator.
Susan Rice, the U.S. ambassador to the United Nations, had been seen as a front-runner for the State Department job, but she withdrew from consideration amid criticism over comments she made on TV talk shows about the killings of Americans in the terrorist attack in Benghazi, Libya, on September 11, 2012.
Kerry immediately became the top candidate. Republicans opposed to a Rice nomination had bandied about Kerry's name, and Senate Minority Leader Mitch McConnell said Kerry would be a "popular choice with the Senate."
But two Republican senators, McCain and Ron Johnson of Wisconsin, raised the Benghazi issue at the hearing Thursday.
Johnson on Wednesday had grilled Clinton about the erroneous initial talking points used to explain the cause of the attack to the public. While it was first said the attack was a spontaneous protest over an anti-Islamic film, the administration later defined the violence as a planned terrorist attack.
"With all due respect, the fact is we had four dead Americans," Clinton said in response to Johnson's repeated questions. "Was it because of a protest or was it because of guys out for a walk one night decided they'd go kill some Americans? What difference, at this point, does it make? It is our job to figure out what happened and do everything we can to prevent it from ever happening again, senator."
Republicans have made the phrase "what difference ... does it make" a talking point, and Johnson seized on it Thursday.
"I think it makes a big difference whether or not the American people have the confidence that the president and the administration is being truthful with them," he said. "So, I guess my question is do you agree with that point? Are you willing to work with me or do you basically kind of agree with Hillary Clinton that that's kind of yesterday's news and let's move on."
But Kerry told Johnson that if he was attempting to "get some daylight between me and Secretary Clinton, that's not going to happen here today."
He offered the possibility that the senator and Clinton are "talking past each other."
"I don't think that was the question," Kerry said. "I think that if your question is, should the American people get the truth and does it matter, Hillary Clinton would say yes and I say yes, but that's not what I think she was referring to. I think what she was referring to was sort of the question of, you know, the sequencing and the timing of how particular information came in with respect to the talking points and the public statements that were made.
"And there was a difference of opinion, in my judgment, as to how you saw that versus how she saw that."
Kerry's stellar reputation