WASHINGTON » Democratic Sen. John Kerry, who unsuccessfully sought the presidency in 2004 and has pined for the job of top diplomat, vaulted to the head of President Barack Obama’s short list of secretary of state candidates after U.N. Ambassador Susan Rice suddenly withdrew from consideration to avoid a contentious confirmation fight with emboldened Republicans.

The exit of Rice and elevation of Kerry shook up Washington on Thursday and was coupled with the potential for even bolder second-term changes in Obama’s national security team next month. Chuck Hagel, a former Republican senator from Nebraska, emerged as the front-runner to serve as defense secretary.

The possible selection of Kerry and Hagel would put two decorated Vietnam War veterans — one Navy, the other Army — at State and the Pentagon.

Official word on replacements for Secretary of State Hillary Rodham Clinton and Defense Secretary Leon Panetta in an Obama Cabinet remake could come as early as next week. The choice of Kerry would open a Massachusetts Senate seat, boosting the prospects for recently defeated Republican Sen. Scott Brown to win back a job in Washington.

Kerry, a senator for nearly three decades and the current Senate Foreign Relations Committee chairman, has won praise from his Senate Democratic and Republican colleagues and should be confirmed easily, if nominated. He has been Obama’s envoy to hot spots such as Afghanistan and Pakistan, the administration’s point man in 2010 on a nuclear arms reduction treaty with Russia and was a stand-in for Republican Mitt Romney during Obama’s debate preparation.

Hagel was a senior member of the Foreign Relations Committee during his years in the Senate. He and Obama became close while they served in the Senate and traveled overseas together. Hagel has irked some in the GOP with his complaints that the party has moved too far to the right and for his endorsements of Democrats, most recently Bob Kerrey in last month’s Nebraska Senate race.

“He’s a combat vet who was wounded twice and understands that the decisions we make here are carried out by our young men and women” in the military, said Sen. Jack Reed, D-R.I., a member of the Armed Services Committee.

It would be highly unusual for Hagel’s political moves to sink his nomination, even in bitterly divided Washington.

But Democrats blamed politics for Rice’s demise as a possible candidate. They clearly insinuated that Republicans who failed to get any traction in using the deadly September attack on the U.S. Consulate in Benghazi, Libya, to derail Obama’s re-election bid instead took down Rice.

She “deserved far more respect than she was shown by certain Senate Republicans,” Senate Majority Leader Harry Reid, D-Nev., said in a blistering statement late Thursday. “Their behavior was a disgrace to the Senate’s tradition of bipartisan cooperation on national security issues and beneath the stature of senators with otherwise distinguished records on national security.

“I hope that moving forward, senators will act based on fact-finding and serving the public interest, not advancing partisan political agendas or settling old scores,” Reid said.

In the past month, Arizona Sen. John McCain, Obama’s Republican presidential rival in 2008, and Sen. Lindsey Graham, R-S.C., had been Rice’s harshest critics, threatening to block her nomination. Sen. Kelly Ayotte, R-N.H., expressed serious concerns. The most damaging blow, however, came from Sen. Susan Collins of Maine, a moderate Republican who voiced doubts not only about Rice in her current job but also when she was a State Department official during the Clinton administration.

All four met with Rice in unusual, private sessions on Capitol Hill where she failed to mollify her critics.

Rice had been widely assailed for a series of Sunday talk show interviews five days after the consulate attack that killed Ambassador Chris Stevens and three other Americans. Relying on intelligence community talking points, she attributed the cause of the raid to widespread protests throughout the Middle East over an anti-Muslim video rather than a terrorist attack by al-Qaida affiliates.

Those much-maligned remarks have dogged Rice since September, but Obama was defiant in defending her. House Democratic women cast the attacks as sexist and racist — Rice is African-American — and some Senate Republicans were nervous about challenging her after an election in which the GOP struggled with black and female voters.

Up until this week, the White House was still privately challenging the record of Rice’s opponents on similar GOP nominations. In Congress, Democrats were more troubled by a steady drip of reports about Rice, from a possible conflict of interest over personal holdings and the Keystone XL pipeline to increased scrutiny about her relationship with the president of Rwanda and the country’s backing of a rebel group in Congo, a charge Rwanda denies.

In a letter to Obama, Rice bowed out, saying that “if nominated, I am now convinced that the confirmation process would be lengthy, disruptive and costly — to you and to our most pressing national and international priorities. That trade-off is simply not worth it to our country.”

She added that she was saddened by the partisanship even before Obama made a nomination, but the country “cannot afford such an irresponsible distraction from the most pressing issues facing the American people.”

Obama bemoaned the relentless Republican criticism in accepting Rice’s decision to step aside.

“While I deeply regret the unfair and misleading attacks on Susan Rice in recent weeks, her decision demonstrates the strength of her character,” the president said.

Kerry is no stranger to the politicization of national security; he was the target of unsubstantiated claims by the Swift Boat Veterans for Truth over his Vietnam record. He acknowledged that experience in his statement Thursday praising Rice.

“As someone who has weathered my share of political attacks and understands on a personal level just how difficult politics can be, I’ve felt for her throughout these last difficult weeks, but I also know that she will continue to serve with great passion and distinction,” he said in a statement.

Tennessee Sen. Bob Corker, who will be the top Republican on the Foreign Relations Committee next year, applauded Rice “for making the right decision to withdraw herself, recognizing the atmosphere and damage a messy confirmation process would cause.”

McCain, Graham, Ayotte and Collins said in statements that they respected Rice’s decision while vowing to press ahead in getting answers about the Benghazi attack.

The White House said Rice would remain U.N. ambassador. She could become national security adviser should Tom Donilon move on to another position, though that is not expected imminently. The security adviser position would not require Senate confirmation.

Associated Press writers Ben Feller and Julie Pace contributed to this report.