The Democratic contest is heating up ahead of “Super Tuesday”
Barack Obama and Hillary Clinton have faced each other in a televised debate as they strive to become the Democratic Party’s presidential candidate. The two held their first head-to-head after John Edwards withdrew from the Democratic race on Wednesday.
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.
The endorsements could give Mr McCain extra momentum ahead of “Super Tuesday”, when 24 states will vote.
The Democratic race is also getting more tense ahead of the potentially decisive series of contests.
Both candidates paid tribute to Mr Edwards in opening statements at Thursday evening’s debate in Los Angeles.
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.
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.
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.
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.
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.