WASHINGTON » President Barack Obama today nominated Massachusetts Sen. John Kerry, one of Washington’s most respected voices on foreign policy, as his next secretary of state.

The move is the first in an expected overhaul of Obama’s national security team heading into his second term.

As the nation’s top diplomat, Kerry will not only be tasked with executing the president’s foreign policy objectives, but will also have a hand in shaping them. The longtime lawmaker has been in lockstep with Obama on issues like nuclear non-proliferation, but ahead of the White House in advocating aggressive policies in Libya, Egypt and elsewhere that the president later embraced.

"He is not going to need a lot of on-the-job training," Obama said, standing alongside Kerry in a Roosevelt Room ceremony. "Few individuals know as many presidents and prime ministers or grasp our foreign policies as firmly as John Kerry."

He is expected to win confirmation easily in the Senate, where he has served since 1985, the last six years as chairman of the Foreign Relations Committee.

Kerry would take the helm at the State Department from Secretary Hillary Rodham Clinton, who has long planned to leave the administration early next year. Clinton is recovering from a concussion sustained in a fall and did not attend the White House event.

In a statement, Clinton said, "John Kerry has been tested — in war, in government, and in diplomacy. Time and again, he has proven his mettle."

Obama settled on Kerry for the job even though it could cause a political problem for Democrats in Massachusetts. Kerry’s move to State would open the Senate seat he has held for five terms, giving Republicans an opportunity to take advantage. Recently defeated GOP Sen. Scott Brown would be his party’s clear favorite in a special election.

Kerry would join a national security team in flux, with Obama expected to choose a new defense secretary and director of the Central Intelligence Agency in the coming weeks.

The 69-year-old Kerry already has deep relationships with many world leaders, formed both during his Senate travels and as an unofficial envoy for Obama. The president has called upon Kerry in particular to diffuse diplomatic disputes in Afghanistan and Pakistan, two countries that will be at the forefront of Obama’s foreign policy agenda early in his second term.

At times, Kerry has been more forward-leaning than Obama on foreign policy issues. He was an early advocate of an international "no-fly zone" over Libya in 2011 and among the first U.S. lawmakers to call for Egyptian strongman Hosni Mubarak to leave power as pro-democracy protests grew. Obama later backed both positions.

Kerry would take over at a State Department grappling with the deaths of the U.S. ambassador to Libya and three other Americans during a September attack on the consulate in Benghazi, Libya. Kerry, during a hearing on the attacks Thursday, hinted at how he would manage U.S. diplomatic personnel working in unstable regions.

"There will always be a tension between the diplomatic imperative to get ‘outside the wire’ and the security standards that require our diplomats to work behind high walls," he said. "Our challenge is to strike a balance between the necessity of the mission, available resources and tolerance for risk."

His only other rival for the job, U.N. Ambassador Susan Rice, faced harsh criticism from congressional Republicans for her initial accounting of the consulate attack. Obama vigorously defended Rice, a close friend and longtime adviser, but GOP senators dug in, threatening to hold up her nomination if the president tapped her for the post.

Rice withdrew her name from consideration last week, making Kerry all but certain to become the nominee. People familiar with the White House’s decision-making said support within the administration was moving toward Kerry even before Rice pulled out.

The son of a diplomat, Kerry was first elected to the Senate in 1984. He is also a decorated Vietnam veteran who was critical of the war effort when he returned to the U.S. He ran for president in 2004, losing a close race to incumbent Republican President George W. Bush.

Obama and Kerry have developed close ties in recent years. It was Kerry, during his 2004 presidential run, who tapped Obama as the party’s convention keynote speaker, a role that thrust the little-known Illinois politician into national prominence.

Kerry served on Obama’s debate preparation team during the 2012 election, playing Republican challenger Mitt Romney in mock debates.

"Nothing brings two people closer together than two weeks of debate prep," Obama joked today. "John, I’m looking forward to working with you rather than debating you."

Kerry is Obama’s first Cabinet appointee following the November election. The president is also mulling replacements for retiring Defense Secretary Leon Panetta and former CIA director David Petraeus, who resigned last month after admitting to an affair with his biographer.

Former Republican Sen. Chuck Hagel of Nebraska is a front-runner for the Pentagon post, but has been dogged by questions about his support for Israel and where he stands on gay rights, with critics calling on him to repudiate a comment in 1998 that a former ambassadorial nominee was "openly, aggressively gay."

Hagel apologized for that comment today.

Former Pentagon official Michele Flournoy and current Deputy Defense Secretary Ashton Carter are also under consideration to replace Panetta. Obama is also considering promoting acting CIA Director Michael Morell or naming White House counterterrorism adviser John Brennan as the nation’s spy chief.

Associated Press writers Donna Cassata in Washington and Steve LeBlanc in Boston contributed to this report.