* Samantha Power faces tough confirmation in Senate
* She could lend more muscle to U.S. human rights push
* Called Hillary Clinton "monster," left 2008 Obama campaign
President Barack Obama has selected as his nominee for U.S. ambassador to the United Nations a Pulitzer Prize-winning author and academic who in 2008 called Hillary Clinton - at the time a political rival of Obama - "a monster."
The White House is due to name Samantha Power to replace current ambassador Susan Rice, who Obama plans to nominate on Wednesday as his national security adviser.
Power, a former White House aide and Harvard professor, is a strong advocate for human rights at a time when the Obama administration is grappling with its response to the civil war in Syria as well as human rights issues in countries such as China and Sudan.
Power caused a stir during the tense contest between Obama and Clinton for the Democratic presidential nomination in the 2008 election. She was serving as an adviser to Obama at the time.
"She is a monster, too - that is off the record - she is stooping to anything," Power was quoted as telling The Scotsman, a British newspaper, referring to Clinton.
"But if you are poor and she is telling you some story about how Obama is going to take your job away, maybe it will be more effective. The amount of deceit she has put forward is really unattractive," Power was quoted as saying.
The remarks prompted her resignation from Obama's campaign team. Obama edged Clinton for the Democratic nomination, won election that November and named Clinton as his top diplomat, a post she held until earlier this year.
The "monster" comment illustrates the close scrutiny that Power's discretion and diplomatic skill will face as the U.S. envoy to the world's leading diplomatic body. It also underscores the power of words for an accomplished former writer who has made other offhand comments likely to draw a second look from critics.
Republicans in the Senate, which must approve her nomination, are likely to give her a rough confirmation hearing.
If confirmed, Power's return to government service would be a comeback after having left the White House earlier this year as senior director for multilateral affairs and human rights on the national security staff.
While that job was relatively low profile, Power was widely reported to have argued for the U.S. decision to intervene militarily in 2011 to support the rebels who eventually toppled long-time Libyan leader Muammar Gaddafi.
No stranger to the power of words, her earlier work as a journalist sent her covering the Balkan wars of the 1990s and conflicts in other countries such as Rwanda, according to her biography on the White House website.
She later won the Pulitzer Prize for her book "A Problem from Hell: America and the Age of Genocide," a study of U.S. policy responses to genocide during the 20th century.
Power is married to legal scholar Cass Sunstein, who until last year headed the White House Office of Information and Regulatory Affairs. He was a friend of Obama dating back to their days on the University of Chicago Law School faculty.