From David Alexander in Washington

When Israeli Foreign Minister Tzipi Livni recently suggested Ehud Olmert should step aside and let her be prime minister, nobody batted an eye over the fact she is a woman. Golda Meir laid that debate to rest in Israel nearly 40 years ago.

Britain had Margaret Thatcher, Pakistan had Benazir Bhutto and India had Indira Gandhi. Women hold high office in a dozen countries around the world, in Asia, Europe, Africa and Latin America, from Chancellor Angela Merkel of Germany to President Gloria Macapagal Arroyo of the Philippines.

So why has the United States - where Senator Hillary Rodham Clinton is a leading contender for the Democratic presidential nomination - never elected a woman to the White House?

"The rules of the game are set up to the advantage of the dominant majority bloc, which is not just Democrat or Republican, but is wealthy white males," argued Andrew Reynolds, a professor at the University of North Carolina.

Women have tended to have two main pathways to high office, Reynolds points put. The first is dynastic, which has been the route in developing countries for many women, such as Gandhi and Bhutto, who were daughters of prime ministers.

The second pathway is seen in wealthier democracies, where more women pursue careers and political positions that act as a springboard to higher office.

Women have more difficulty being elected in presidential systems, insists Ann Gordon, a political scientist who co-edited the book Anticipating Madame President.

It's a piece of the puzzle, she says, but she also sees greater challenges.

"The single biggest obstacle in this country is that voters are uncomfortable with the idea of a woman who is commander in chief," Gordon said. "That's due to gender stereotypes."

Clinton, a New York Democrat, has tried to address the issue by sitting on the Senate Armed Services Committee and taking a strong stance on terrorism during debates, says Susan Carroll, a senior scholar at the Centre for American Women and Politics.

Women also have been hampered by media coverage, which often focuses on their viability as candidates rather than their message. And structural issues have made it difficult for women as well as minorities to pursue the presidency.

"One of the biggest barriers is simply that we haven't had one," Carroll said. "People have to get used to the idea."

Women have been seeking the US presidency since Victoria Clafin Woodhull ran on the Equal Rights Party ticket against Ulysses S Grant and Horace Greeley in 1872.

Despite the difficulties, Gordon said the United States would eventually elect a woman.

"It's not a question of if but when," she said. "And yeah, I think the time is now."