Washington  - She was Barack Obama's chief challenger in the dramatic two-year neck-and-neck race for the Democratic presidential nomination.

Over and over again, Senator Hillary Rodham Clinton, 61, charged that then-senator Obama, 47, lacked political experience. But in the end, Clinton conceded defeat, opening the way for the first African- American president.

Now the New York senator stands in the limelight as president- elect Obama's nominee for the top diplomatic post as secretary of state, an announcement Obama made Monday in Chicago after weeks of speculation that she was the frontrunner for the job.

After racking up at least 18 million votes for the Democratic nomination, the former first lady was reluctant to give up the race and many of her supporters threatened to boycott Obama. Defeat and concession are rare words in Clinton's vocabulary. Ambitious, determined and aggressive - those are the qualities that drive the wife of former president Bill Clinton.

She has also been a divisive figure, with critics even within Democratic ranks considering her cold and calculating.

The Chicago-born senator learned to fight early in life. She grew up in a strict household, ruled with iron discipline by her father, according to one Clinton biographer. She met Bill Clinton as a student, moving with him to Arkansas, where he became governor, and then into the White House in 1993.

The White House years were hard. Married to a well-known skirt- chaser, she stood by him during his ribald sex affair with intern Monica Lewinsky. The couple faced aggressive investigations by a Republican-controlled Congress that impeached Bill Clinton over the Lewinsky affair - but did not remove him from office.

As first lady, she sought centre stage by launching an effort to reform the country's health care system, but failed miserably. The push for universal medical insurance remained a cornerstone of Clinton's Senate career, which began in 2001, the same year the power couple left the White House.

In nominating his rival as the top US diplomat, Obama said he was banking on Clinton's "extraordinary intelligence and a remarkable work ethic" to send a signal to "friend and foe of the seriousness of my commitment to renew American diplomacy and restore our alliances."

Clinton said after her nomination that the decision to leave the Senate was "very difficult," but she realized how important it was to support US troops and foreign and civil service officials who are "working hard to promote and protect our interests around the world."

She said the "daunting tasks ahead" for the country - a troubled economy and two wars - convinced her to accept the job.

She said the American people, by electing Obama, had demanded a "new effort to renew America's standing in the world as a force for positive change" - an effort she said she would promote with all tools available.

"We must ... build a future with more partners and fewer adversaries, more opportunities and fewer dangers for all who seek freedom, peace, and prosperity," she said. (dpa)