U.S. presidential primaries begin Tuesday in Iowa, which holds the nation's first nomination contest. While there is no major Democratic challenger to Barack Obama, the incumbent president, the Republican nomination race has so far featured historical volatility with no clear front-runner in sight yet. Following is a brief introduction to the current major contenders in the race.

DEMOCRATIC CANDIDATES:

On the Democratic side, Obama is likely to win party nomination although a few others have registered in some states to challenge him in the primary process. Those contenders, with very low name recognition, are not expected to bring any trouble to the president.

Obama, 50, is the first African-American president in the U.S. history. He previously served as a senator from Illinois, from January 2005 until he resigned following his victory in the 2008 presidential election.

The biggest challenge for Obama's reelection campaign is the nation's economy. With lackluster economic growth and high jobless rates, it's unclear if American voters are willing to give him another four years to get the nation back on track.

Still, the incumbent president has some strong points, including popularity among African-American and Latino voters, as well as unparalleled fund-raising capability and sophisticated campaign network across the country.

Obama will have a good shot at a second term if the economy improves somewhat in a few months ahead of November or Republicans nominate someone who is uncompetitive or ideologically extreme, analysts said.

REPUBLICAN CANDIDATES:

The battle is more entangled on the Republican side. On the national stage, former Massachusetts Governor Mitt Romney and former House Speaker Newt Gingrich are neck-and-neck as top-tier candidates in major opinion polls. But in some key battlegrounds like Iowa, House Representative Ron Paul of Texas and former Senator Rick Santorum of Pennsylvania have also shown some strength.

MITT ROMNEY-- The former Massachusetts governor has been pretty stable since entering the race, in contrast to the rapid rise and fall of his colleagues. His poll numbers have largely stayed at mid-20s, making him a top-tier candidate throughout the process to date.

Romney, 64, has both experiences in private business and in executive office as governor. He served as CEO of Bain & Company and brought it out of crisis. During his governorship from 2003 to 2007, he presided over a series of spending cuts in Massachusetts that eliminated a projected 3-billion-dollar deficit.

This is Romney's second run for the White House after an unsuccessful bid in 2008. This time around, he faces similar challenges he did four years ago -- conservative skepticism of his convictions and his Mormon faith, controversy over his embrace of a healthcare program in Massachusetts similar to that of the Obama administration.

NEWT GINGRICH-- Riding strong debate performances, the former House speaker rose to the top of the field in late November, but seemed to have lost momentum in recent weeks with dropping poll numbers.

Gingrich, 68, is considered one of his party's most creative thinkers. As a co-author of the 1994 Contract with America, he was in the forefront of the Republican Party's dramatic success in that year's midterm elections and subsequently was elected speaker of the House.

However, following Republican losses in the 1998 midterm elections, Gingrich resigned both his speakership and his congressional seat.

The biggest challenge for him is the huge "baggage" he is carrying -- two divorces, previous consultancy for Freddie Mac, among others.

RON PAUL-- The Texas congressman has recently shown his strength particularly in Iowa, where he sits atop the field together with Romney and Santorum. Despite his momentum, conventional wisdom still sees his White House bid as long-shot given some of his outside-mainstream views.

Paul, 76, gained prominence for his libertarian positions on many political and social issues, often at odds with both Republican and Democratic leaders. He opposes military intervention in foreign countries and is against both wars in Iraq and Afghanistan.

This is the third time for Paul to stand for presidency. He first ran in 1988 as the nominee of the Libertarian Party and again in 2008 as a candidate for the Republican nomination but failed.

RICK SANTORUM-- The former Pennsylvania senator, who has staked his campaign on a strong showing in Iowa, began to reap the fruits. His poll numbers in the Hawkeye state have risen to almost tie with Romney and Paul after spending most of his time and money there.

Santorum, 53, was first elected to the House of Representatives in 1990, and four years later was elected to the Senate. He was the chairman of the Senate Republican Conference, the third-ranking Senate Republican, from 2001 until his leave in 2007.

He is considered both a social and fiscal conservative. He enjoys strong support among social conservatives in Iowa thanks to his opposition to abortion, gay marriage and embryonic stem cell research.

But even with a good showing in Iowa, Santorum faces significant challenges ahead. Among them are the little name recognition he has beyond Pennsylvania, as well as the poor campaign networks he has established beyond Iowa.

RICK PERRY-- The former Texas governor, who surged to the top of the field soon after launching his bid, has sunk to second-tier status, a decline partly attributed to weak debate performances.

Perry, 61, assumed the governorship of Texas in December 2000 when then-governor George W. Bush resigned to become the president. With a tenure in office to date of more than 11 years, he is one of the longest-serving governors in the U.S. history.

Perry is popular with both Tea Party supporters and conservative evangelicals, two voting blocs with much influence in campaigns. He is casting himself as the "only outsider" in the GOP field amid strong anti-Washington sentiment.

MICHELE BACHMANN-- The congresswoman from Minnesota was seen as a promising star in the GOP field but lost her spotlight soon after Perry's entry into the race. She is now considered a second-tier candidate and is competing with Perry and Santorum for the conservative votes.

Elected to Congress in 2006, Bachmann, 55, was the founder of the House Tea Party caucus. She is consistently conservative on both fiscal and social issues.

Tea Party supporters, Bachmann's biggest asset, is also believed to be her biggest liability, as her views are sometimes considered to be too extreme to woo Republican establishment and independent voters.

JON HUNTSMAN-- The former Utah governor has mostly failed to gain traction in this campaign except in New Hampshire, where he is betting on a strong showing to stay alive.

Huntsman, 51, distinguishes from the rest of the field with significant foreign policy experience. He served in diplomatic positions in the two Bush administrations and as U.S. ambassador to China under President Obama.

He is a moderate, which works against him in the conservative-dominated Republican primaries. He also faces challenges from his Mormon faith and experience of working for a Democratic president.