( BBC ) - Barack Obama and Hillary Clinton are facing each other in a televised debate as they strive to become the Democratic Party's presidential candidate.
The two go 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.
It is unclear whether Mr Obama or Mrs Clinton will benefit most from the withdrawal of Mr Edwards, says the BBC's Kim Ghattas in Washington.
He has not endorsed either of them yet and is likely to be courted intensely by both camps, our correspondent says.
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.