Presidential hopefuls are campaigning across the US as a series of primaries in key battleground states nears.
Crucial party nomination battles remain in four states this month
The next focus for the Democrats is the Nevada caucus on 19 January, set to be a head-to-head test of support between Hillary Clinton and Barack Obama.
The Republicans, whose race is between several contenders, are targeting Michigan and South Carolina, where they will hold a TV debate later in the day.
Another crucial primary will take place at the end of the month in Florida.
The hopefuls are aiming to build momentum before 22 states nominate their preferred candidate on 5 February, dubbed Super Tuesday, to run in November's presidential election.
The style of campaigning seen in the first two contests in Iowa and New Hampshire, where candidates wooed voters face-to-face, will now change, analysts say.
It will now be a broader national campaign driven by state-hopping plane trips and big-money media advertisements.
Mr Obama has been given the backing of the Democrats' 2004 presidential nominee, Massachusetts Senator John Kerry, who lost to President George W Bush.
Mr Kerry told a rally at the College of Charleston, in South Carolina, that he was confident "Barack Obama can be, will be and should be the next president of the United States".
Mr Kerry said there were other candidates in the race with whom he had worked and whom he respected.
But, he added: "I believe more than anyone else, Barack Obama can help our country turn the page and get the America moving by uniting and ending the division we have faced."
Mr Obama then addressed the crowds, who cheered and waved banners as he repeated his campaign message of "change we can believe in" and hope for a united America.
Mr Kerry gave Mr Obama's career a boost in 2004 when he chose the young Illinois state senator to give the keynote speech at the Democrat National Convention. A year later, Mr Obama was elected to the US Senate.
More than half of South Carolina's Democratic voters are African-American and, say analysts, are likely to vote for the 46-year-old Illinois senator, who aims to become the first black US president.
Wide open race
The Republican contest has been blown wide open after 71-year-old Arizona Senator John McCain's comeback triumph in New Hampshire over Mitt Romney on Tuesday.
The two rivals, who campaigned on Wednesday in Michigan, will head south on Thursday to face other Republican White House hopefuls in a TV debate at Myrtle Beach, South Carolina at 2100 local time (0200GMT).
Mr Romney - who aims to become the nation's first Mormon president - must win Michigan, the state of his birth, to resurrect his high-spending but faltering campaign, correspondents say.
Fred Thompson, the former Tennessee senator, is in South Carolina also struggling to mount a comeback, while ex-Arkansas governor Mike Huckabee is seeking to woo the state's evangelical conservatives.
Mr Huckabee was the Republican winner in Iowa but came third in New Hampshire.
Further south, former New York Mayor Rudolph Giuliani has been campaigning in Florida.
Meanwhile, New Mexico Governor Bill Richardson has announced he is quitting the Democratic race, after trailing in the New Hampshire primary and in last week's Iowa caucuses.
"I gave this race the best I have," he told supporters in Santa Fe, New Mexico, on Thursday.
Gov Richardson would have been the first Hispanic US president if elected, but his campaign failed to gain momentum.
His best chance was expected to be in Nevada, which has a large Hispanic population, but polls suggested he was lagging well behind the frontrunners.
Two large trade unions in Nevada have announced that they are endorsing Mr Obama's candidacy.
While Hillary Clinton spent much of Wednesday at her home in upstate New York conferring with advisors, Mr Obama campaigned in New Jersey.
The two Democrat front-runners were nearly neck-and-neck on fundraising after the Clinton camp reported its campaign had been boosted by $24m (£12m) in the last three months of 2007, while the Obama team received more than $22m (£11m) over the same period.
Former senator John Edwards, who was born in South Carolina, has shrugged off his third-place finish in New Hampshire and vowed to remain in the Democratic race.
He said he respected the decision of Mr Kerry - for whom he was the running mate in the 2004 presidential race - to endorse Mr Obama.