WASHINGTON (AP) — A moderate Republican senator, vital to any White House hopes of getting U.N. Ambassador Susan Rice confirmed as secretary of state, said she couldn’t back any nomination until more questions are answered about the Sept. 11 attack on the U.S. Consulate in Benghazi, Libya.

In a fresh suggestion of eroding Republican support for Rice, Sen. Susan Collins on Wednesday also raised concerns about Rice’s State Department role during the 1998 U.S. Embassy bombing in Kenya. Collins, emerging from a 90-minute, closed-door meeting with the ambassador, questioned what Rice, the assistant secretary of state for African Affairs in the Clinton administration, knew about requests for enhanced embassy security before the Nairobi truck bombing.

Obama has not announced his nominee to replace Secretary of State Hillary Rodham Clinton, but Rice is widely seen as his top pick. Although Democrats will have 55 votes in the next Senate, the president would need the support of five Republicans to avoid delaying tactics that could scuttle the nomination. Collins would be a prime candidate to help avoid that scenario.

Pressed on how she would vote if Obama names Rice to succeed Clinton, Collins said, “I would need to have additional information before I could support her nomination.”

At issue is the explanation that Rice offered in a series of talk show appearances five days after the Benghazi attack that killed U.S. Ambassador Chris Stevens and three other Americans.

Rice conceded to the senators that her initial account — that a spontaneous demonstration over an anti-Muslim video produced in the U.S. triggered it — was wrong, but she insisted she had not been trying to mislead the American people.

Obama came to Rice’s defense during a Cabinet meeting, calling her “extraordinary” and saying he couldn’t be prouder of the job she has done as U.N. ambassador. Cabinet members joined Obama in applauding Rice, who attended the meeting. Obama has not named a replacement for Clinton, who has said she intends to step down soon.

At the State Department, Clinton was asked about her possible replacement.

“Susan Rice has done a great job as our ambassador to the United Nations,” Clinton said. “Of course, this decision about my successor is up to the president, but I am very happy he has the opportunity with a second term to make a decision.”

The misgivings from Collins, the top Republican on the Homeland Security Committee, came one day after three other Republican senators said they would try to block Rice’s nomination. Sens. John McCain, Lindsey Graham and Kelly Ayottee said they were more troubled than ever by Rice’s answers on Libya.

In an unusual move, Rice and acting CIA Director Michael Morell have held two days of private meetings with Republican senators in hopes of assuaging their concerns. Privately, Senate Republicans said they had hoped the conversations would quiet the criticism as they want to avoid the spectacle of a postelection challenge to a female African-American nominee.

Instead, the sessions have cast further doubt on her chances for the top State Department job and increased the likelihood of a protracted fight if Obama does choose her.

Collins said she was troubled by Rice’s “political role” in downplaying the Libya attack as a spontaneous demonstration over an anti-Muslim video rather than a terrorist attack by al-Qaida affiliates.

Rice has said she was relying on talking points provided by U.S. intelligence.

Introducing another issue certain to be fodder for any confirmation battle, Collins said she pressed Rice about security at the U.S. Embassy in Nairobi in 1998 when a truck bomb was set off outside the facility, killing more than 200 Kenyans and 12 Americans.

“What troubles me so much is the Benghazi attack in many ways echoes the attacks on those embassies in 1998 when Susan Rice was head of the African region for our State Department,” Collins told reporters after the meeting. “In both cases, the ambassador begged for additional security.”

Collins said Rice told her she was not involved directly in turning down the request for improved security. The Maine senator said that in light of Rice’s position, she had to be aware of the general threats and U.S. Ambassador Prudence Bushnell’s requests for security upgrades in Kenya.

Review boards headed by former Joint Chiefs of Staff chairman Adm. William J. Crowe after the Aug. 7, 1998, bombings of the U.S. embassies in Kenya and Tanzania did not find reasonable cause that any U.S. employee breached his duty in connection with the bombings. Rice was not blamed.

However, Crowe said the boards believed there was a “collective failure” by several administrations and Congress over a decade to invest adequately to shore up vulnerable U.S. diplomatic missions around the world.

Although Rice has emerged as the front-runner for the top job at State, Senate Foreign Relations Committee Chairman John Kerry, passed over for the job in 2008, is considered a strong alternative.

In a clear message to the White House, Collins said Kerry, a former presidential candidate, would have no problem winning Senate confirmation.

Sen. Bob Corker, who is in line to become the top Republican on the Foreign Relations Committee, was more circumspect about Rice’s chances after his own meeting with the ambassador.

The Republican senator suggested that Obama “take a deep breath and nominate the person he really believes is the very best person for secretary of state, regardless of relationships.”

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Associated Press writer Matthew Lee contributed to this report.

Copyright 2012 The Associated Press.