Now the question is: Can he win?
Most said that if California voters decide to dump Gov. Gray Davis in the state's first-ever recall election, Schwarzenegger stands an excellent chance of finishing first in the race to replace him.
"He's not a sure thing, but he's a natural politician, a very intelligent man and a very credible candidate," said John Pitney, a political scientist at Claremont McKenna College in Claremont, Calif.
San Francisco Mayor Willie Brown, a Democrat and nominal Davis supporter, said that "Democrats are going to have a very difficult time defeating Arnold."
Republicans are closing ranks behind the movie star, while "we are in trouble on the Democratic side with Governor Davis," Brown said on CNN.
Davis said late in the day that California voters would answer the question: Will Schwarzenegger become the next governor?
"The Terminator may be back. He may not be back. The people will make that choice," the governor told reporters.
On a day of fast-moving developments, it was clear that Schwarzenegger's entry had triggered shock waves that rolled through both parties. Among them:
The decisions by Lt. Gov. Cruz Bustamante and Insurance Commissioner John Garamendi further weakened Davis' hold on his office, because keeping credible Democratic choices off the ballot had been central to his strategy for beating the recall.
Riordan issued no immediate public statement, however.
That Republican could be Schwarzenegger.
While his candidacy has just begun, a number of factors appear to be working in his favor.