Mr McCain is to appear alongside the governor of California later.The Republican front-runner has had a good week, winning the Florida primary and then securing the support of Rudy Giuliani, who has now quit the race.
Meanwhile, the two leading Democratic candidates will go head-to-head in a debate after John Edwards withdrew.Barack Obama and Hillary Clinton are left battling for their party's nomination after Mr Edwards, who had failed to win any of the Democratic nomination contests held so far, quit the race on Wednesday.
In the Republican race, Mr McCain has emerged as the front-runner, pulling ahead of main rival, Mitt Romney.His win in the Florida primary election means he has secured more of the delegates who will choose the Republican Party's presidential candidate in September.
He also has some high-profile backing.Mr Giuliani, the former New York mayor, pulled out of the Republican race after disappointing results in Florida, announcing his support for his friend John McCain, whom he described as "an American hero".
Mr Schwarzenegger's endorsement will also be important for Mr McCain, an Arizona Senator.It may improve his chances of winning the California primary - one of the many to be held next week on "Super Tuesday".
Whichever Republican takes California will win nearly 15% of all the delegates he needs to secure the party's nomination.
A Republican debate on Wednesday night 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.
Former Massachusetts Governor Mr Romney accused Mr McCain of "dirty tricks" for accusing him - just before Tuesday's Florida primary - of supporting a date for withdrawing troops from Iraq.
"It's offensive to me that someone would suggest that I have," said Mr Romney.Mr Romney said Mr McCain was out of the conservative mainstream, having twice voted against President George W Bush's tax cuts, which the Republican establishment had embraced.
Mr McCain countered that he was proud of his conservative credentials, and questioned Mr Romney's administrative record in Massachusetts."His job creation was the third worst in the country," said the Arizona senator.
The contest to be the Democratic candidate is also getting more tense ahead of the potentially decisive Super Tuesday.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.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.