The two will appear at a news conference after touring a Los Angeles-based solar energy company and the governor will make his endorsement official, his senior aides confirmed.
Schwarzenegger's endorsement of McCain is yet another setback for Mitt Romney, the former Massachusetts governor who saw Florida slip from his grasp Tuesday after McCain rolled up the support of that state's two top elected Republicans, Gov. Charlie Crist and Sen. Mel Martinez.
His strategy in tatters, Romney plans to offer himself as the conservative alternative to McCain as he pushes ahead in hopes of winning enough delegates to topple the Arizona senator when 21 states vote in the Republican contest on Tuesday.
Schwarzenegger's move comes as McCain plows toward the nomination, the only Republican candidate to have won three hotly contested primaries - New Hampshire, South Carolina and Florida - since voting began earlier this month.
"Governor Schwarzenegger is an exceptional governor and we are honored that he has decided to endorse Senator McCain, and look forward to the event tomorrow," said Steve Schmidt, a senior McCain adviser who managed Schwarzenegger's 2006 campaign.
Schwarzenegger sat in the audience here Wednesday as McCain and Romney shared a debate stage with rivals Mike Huckabee and Ron Paul at the Ronald Reagan Presidential Library.
McCain, the four-term senator, is running strongly ahead of all three rivals in California, which offers a whopping 170 delegates to the Republican nominating convention. Candidates secure three delegates for each of the state's 53 congressional districts they win in the primary, in which only Republicans can vote.
The ultimate effect of Schwarzenegger's endorsement is unclear.
The celebrity governor and former actor is universally known in the state, and his political network certainly will be helpful to McCain, who has virtually no organized effort in California after his candidacy nearly collapsed last summer. The actor-turned-governor also is a prolific fundraiser.
But Schwarzenegger has a strained relationship with some conservatives in his own party and McCain, himself, is fighting to convince GOP rank-and-file that he's committed to conservative values. Schwarzenegger's nod could exacerbate concerns about McCain among the party establishment.
Schwarzenegger also is taking heat from state Republicans who argue he's been too willing to bend to the wishes of the Democratic-controlled Legislature. At the same time, California faces a $14.5 billion budget deficit over the next year-and-half, and the governor has rankled the state's powerful education lobby with his proposal to cut spending by 10 percent from state agencies to deal with the financial crisis.
McCain and Schwarzenegger have been friends for years, and the two share a bond over their work on global warming issues as well as their similar independent streaks. Aides say Schwarzenegger long has respected McCain's push to eliminate wasteful spending in Washington, protect the environment and fix a broken immigration system.
The governor offered high praise of McCain throughout the campaign, calling him a "great senator" and "very good friend," and the two appeared together at the Port of Los Angeles last year. "We share common philosophy and goals for this country," McCain said at the time.
But Schwarzenegger always has stopped short of endorsing McCain, given that another friend, former New York Mayor Rudy Giuliani, also was in the race.
Earlier this month, Schwarzenegger told reporters he would not make an endorsement in the GOP primary, saying then: "It doesn't help me, and it doesn't help the state of California." But senior advisers say Giuliani's departure from the race Wednesday changed the dynamics of the decision for Schwarzenegger, and he decided to go ahead with the endorsement as it was clear that Giuliani's candidacy was over.
