Barack Obama and Hillary Clinton have faced each other in a televised debate as they strive to become the Democratic Party's presidential candidate.
Barack Obama and Hillary Clinton have faced each other in a televised debate days ahead of a string of key primary contests in the White House race.
The two held their first head-to-head after John Edwards withdrew from the Democratic race on Wednesday.
The contest to become Democratic presidential candidate has been getting more tense ahead of votes in 24 states on "Super Tuesday" next week.
In the Republican race, California Governor Arnold Schwarzenegger added his support to John McCain's bid.
They debated head-to-head after John Edwards withdrew from the race.
Rudy Giuliani, who pulled out of the Republican contest this week, has also said he is backing Mr McCain.
Both refused to rule out the idea of running for office as presidential candidate and running mate.
The endorsements could give Mr McCain extra momentum ahead of "Super Tuesday", when 24 states will vote.
Mr Obama has seen his campaign funds swell since the start of the year, with $32m of contributions reported in January.
The Democratic race is also getting more tense ahead of the potentially decisive series of contests.
He is now running TV advertisements in 20 of the states which will be voting in primaries on Tuesday.
Both candidates paid tribute to Mr Edwards in opening statements at Thursday evening's debate in Los Angeles.
His opponent has yet to released figures for the past month, but the Clinton campaign says it raised 26.8m in the last three months of 2007.
Cordial
Mr Obama said the US was facing a "defining moment".
Mr Obama said the US was facing a "defining moment".
"What is at stake right now is whether we are looking backwards or forwards," he said.
Senator Clinton gave a number of different answers over the course of six weeks on [illegal immigrants and driving licences] Barack Obama
"What is at stake right now is whether we are looking backwards or whether we are looking forwards. I think it is the past versus the future."
Mrs Clinton said that the Republican administration of President George W Bush had created a "stack of problems".
Mrs Clinton said that the Republican administration of President George W Bush had created a "stack of problems".
"It is imperative that we have a president who can begin to solve these problems," she said.
"It is imperative that we have a president, starting on day one, who can begin to solve our problems," she said.
Verbal jousting
The debate was mostly cordial in tone.
On the Republican side, it is thought that Mr Schwarzenegger's backing may improve Mr McCain's chances of winning the California primary - one of the many to be held on Super Tuesday.
"I was friends with Hillary Clinton before we started this campaign; I will be friends with Hillary Clinton after this campaign is over," said Mr Obama.
Mrs Clinton noted that "the differences between Barack and I pale in comparison to the differences that we have with Republicans".
'Gravitas'
But there were also moments of tension, including during a discussion about the Iraq war.
I sponsored immigration reform before Barack came to the Senate Hillary Clinton
Mrs Clinton said that she had "the necessary credentials and gravitas" to lead the country in withdrawing from Iraq without endangering U.S. forces or further destabilizing the area.
Mr Obama responded: "Senator Clinton mentioned the issue of gravitas and judgment. I think it is much easier for us to have the argument when we have a nominee who says `I always thought this was a bad idea. This was a bad strategy.' It was not just a problem of execution."
Another pointed exchange came on the subject of whether illegal immigrants should be able to obtain driver's licences.
Mr Obama supports such a policy while Mrs Clinton at first backed it and now opposes it.
"Senator Clinton gave a number of different answers over the course of six weeks on this," Obama said, turning to Clinton.
Mrs Clinton called the controversy "a diversion" from efforts to come up with comprehensive immigration reform.
"I sponsored immigration reform before Barack came to the Senate," she said.
McCain endorsements
In the Republican race, California Governor Arnold Schwarzenegger added his support to John McCain's bid.
Rudy Giuliani, who pulled out of the Republican contest this week, has also said he is backing Mr McCain.
It is thought that Mr Schwarzenegger's backing may improve Mr McCain's chances of winning the California primary - one of the many to be held on Super Tuesday.
The Republicans held their own televised debate on Wednesday night.
The Republicans held their own televised debate on Wednesday night.
It was dominated by verbal jousting between Mr Romney and Mr McCain, with the two others in the race - Mike Huckabee and Ron Paul, who are both trailing - struggling to be heard.
It was dominated by verbal jousting between Mr Romney and Mr McCain, with the two others in the race - Mike Huckabee and Ron Paul, who are both trailing - struggling to be heard.
Mr McCain has emerged as the front-runner after winning the Florida primary.
Mr McCain has emerged as the front-runner after winning the Florida primary.
Meanwhile, Thursday was the deadline for candidates to file details of their campaign finances, up to the end of 2007, promising to shed some light on their multi-million dollar spending.
Will you be voting on Super Tuesday? Will these endorsements influence you at all? Send us your comments using the form below.
Will you be voting on Super Tuesday? Will these endorsements influence you at all? Send us your comments using the form below.
