The protagonists in the race

Barack Obama, 47, was born in Honolulu, Hawaii, and educated at the Columbia University and Harvard Law School. He is married to Michelle Robinson and have two daughters.

He is the son of a Kenyan father and a white American mother. His father, Barack Obama Sr, married his mother, Ann Dunham and separated two years after Mr Obama was born. His father ultimately returned to Kenya, where he became a noted economist. He died in a car accident in 1982.

Mr Obama's mother's second marriage was to an Indonesian man named Lolo Soetoro. The family moved to Indonesia and Mr Obama remained there until he was 10 when he moved back to Hawaii and lived with his grandparents while studying on a scholarship at the elite Punahou Academy.

He has seven half-brothers and sisters in Kenya from his father's side of the family and a half-sister, Maya Soetoro-Ng, from his mother's side.

He landed a job in Chicago in 1985 as an organiser for Developing Communities Project, a church-based group seeking to improve living conditions in poor neighbourhoods.

Mr Obama won a seat in the Illinois state Senate in 1996. He won a heavily contested US Senate seat in 2004, carrying 53 per cent of the Democratic primary vote in an eight-candidate race. He easily won the general election as well.

The non-partisan National Journal ranked him as the most liberal member of the US Senate early this year based on his voting record last year. He was ranked 10th most liberal in 2006 and 16th most liberal in 2005.

Joe Biden

Joe Biden, 65, was born in Scranton, Pennsylvania, and obtained the Bachelor's degree from University of Delaware and law degree from Syracuse University.

He is married to Jill Tracy Jacobs. He has two sons from first marriage and one daughter from his second marriage. Mr Biden married Neilia Hunter in 1966 while he was in law school. The couple had three children - a daughter and two sons.

Mr Biden was elected in 1972 to his first term in the US Senate. Six weeks later, his wife and infant daughter were killed in a car accident that also left his two sons critically injured. He almost resigned the Senate post but was persuaded not to by the majority leader. He took the oath of office at the bedside of one of his sons.

He was a single parent for five years, commuting to and from Washington every day, an hour-and-a-half trip each way, a practice he continues. He met Jill Tracy Jacobs, a teacher, in 1975 and the two married in 1977. They have a daughter. Mr Biden's sons recovered.

Mr Biden graduated near the bottom of his class from both the University of Delaware and Syracuse University law school.

He was elected to the New Castle County Council in 1970 and served for two years before being elected to the US Senate in 1972.

Mr Biden is chairman of the Senate Foreign Relations Committee and a senior member of the Judiciary Committee.

During the war in Bosnia, he became one of the strongest advocates for action to protect Bosnian Muslims, from lifting the arms embargo to Nato air strikes to war crimes prosecutions.

As the Bush administration moved towards war with Iraq, he said the US probably had no choice but to remove Iraqi leader Saddam Hussein.

John McCain

If elected, John McCain, who turned 72 on August 29, would be the oldest first-term US President.

Mr McCain is the son and grandson of four-star admirals and followed in their footsteps by attending the US Naval Academy, where he led what he called a group of troublemakers.

Shot down during a combat mission over Vietnam in 1967, he spent five-and-a-half years as a prisoner of war, including two years in solitary confinement, and was subjected to beatings and torture. In the Senate, he has been a critic of harsh interrogation techniques such as "waterboarding," the simulated drowning of terrorism suspects. Mr McCain turned down a chance to leave prison before comrades who had entered earlier.

In the past 15 years, he has been treated four times for skin cancer. The most serious case was in 2000 when he underwent surgery on his face for melanoma, leaving him with a bulge and a long scar on the left side of his face. He has had regular medical checks and been cancer free since then.

First elected to the US House of Representatives in 1982, he won the first of four Senate terms in 1986.

Mr McCain has been married to Cindy Hensley McCain since 1980. He was married to his first wife Carol, from 1965-1980. He has seven children.

Sarah Palin

Sarah Palin, 44, was born in Sandpoint, Idaho, and educated at the University of Idaho. She is married to Todd Palin and have two sons and three daughters.

Ms Palin moved to Alaska with her parents, a teacher and school secretary, when she was an infant.

She was a member of the state champion girls basketball team, where she earned the nickname "Sarah Barracuda" for her aggressive play. She was crowned Miss Wasilla in a 1982 beauty pageant and later competed in the Miss Alaska contest. She graduated in 1987 from the University of Idaho with a degree in journalism and political science and eloped with her high school boyfriend in 1988 to avoid the cost of a wedding.

After her graduation and marriage, Ms Palin worked as a television sports reporter from 1987 to 1989. She also worked in the family's commercial fishing business and was the owner of a snowmobile, watercraft and all-terrain vehicle business.

She was elected to the Wasilla City Council in 1992. She served two terms on the council and was elected mayor for two terms.

Ms Palin ran for lieutenant governor in 2002 but lost. After actively campaigning for the Republican ticket under Frank Murkowski, she was appointed to the panel that regulates the state's oil and gas industry.

She ran against the governor in 2006, defeated him in the primary and then defeated a former Democratic governor in the general election.

As governor, she has worked on ethics reform, sought to reduce state spending and promoted a deal that would offer $500 million in seed money for the construction of a natural gas pipeline.