Four candidates remain in the race to determine the U.S. Republican Party's challenger to Democratic President Barack Obama.

Mitt Romney, a former governor of Massachusetts, had a successful career as a businessman before becoming a politician.

This is his second time running for the Republican presidential nomination after an unsuccessful bid in 2008.

This time, the multimillionaire investor has consistently polled at the top of the field of candidates, but he is not always the front-runner.

Questions about Romney's personal wealth have dogged his campaign, and he has also faced scrutiny for his membership in the Mormon Church — a religious group that believes in the Christian Bible but also follows teachings from their founder and a holy book separate from the Bible.

The 64-year-old Romney, considered one of the more moderate Republican candidates, comes from a political family. His father was the governor of Michigan and ran for the Republican Party's presidential nomination in 1968.

Newt Gingrich is a former speaker of the U.S. House of Representatives whose career has been plagued by controversy.

Gingrich is the only House speaker disciplined for ethics violations, an incident that contributed to his abrupt resignation from the House in 1999.

The 68-year-old Gingrich has also faced criticism for his personal life. He has been married three times, with his second and third marriages stemming from extramarital affairs. His push for President Bill Clinton's impeachment after Mr. Clinton lied about an affair with a White House intern led some to label Gingrich a hypocrite.

Despite scrutiny of his past, Gingrich has seen sporadic surges in public support during the race for the Republican presidential nomination.

Analysts say Gingrich is an intelligent and experienced politician. He often earns praise for his debate performances, although he has been criticized for changing his positions over the years, including on climate change.

Rick Santorum spent 16 years in Congress representing his home state of Pennsylvania, and often draws on his Catholic faith when campaigning.

The 53-year-old social conservative served in both houses of Congress, first in the House of Representatives, then in the Senate.

He says he is committed to strengthening the American family and the values of the country.

After months of languishing near the bottom of the polls, Santorum saw a surge in support ahead of the first nominating contest on January 3 in Iowa.

Initially, officials said he came in second in the Iowa caucuses, just eight votes behind Romney, but two weeks later, an updated tally revealed Santorum actually received the most votes.

Ron Paul, a Texas physician and House lawmaker, is a proponent of libertarian philosophies, which emphasize individual freedom and limited government.

At 76, Paul is the oldest of the Republican candidates. He is well-known for his anti-war stance and is popular among younger voters and supporters of the conservative Tea Party movement.

Paul has run for president twice before, in 1988 as a Libertarian candidate and in 2008 as a contender for the Republican nomination.

He dropped out of the 2008 race and founded an advocacy group, Campaign for Liberty, which endorses strict adherence to the U.S. Constitution.