Lt. Gov. Gray Davis and Rep. Jane Harman denounced Checchi's TV advertising blitz and claimed it had degraded California politics.
"Al Checchi has taken this campaign down a path none of us wanted to go," Davis said. "You could do a great service to the people of California and finish on a high note by focusing on your merits as a candidate rather than the alleged deficiencies of the rest of us."
Harman ripped into Checchi by saying his "divisive" ads suggest he would be a divisive governor.
"Too many macho politicians say, 'Do it my way, or not at all, and if you disagree with me, I'll tell the press something naughty you did when you were 12,"' she said.
Checchi opened the forum with an upbeat pledge to improve public education the topic that dominated the debate then smiled as his fellow Democrats lobbed charges at him.
Attorney General Dan Lungren, the presumptive Republican nominee, stayed above the fray, urging the eventual Democratic nominee to "start engaging in ideas early. ... Let's say issues matter."
For weeks, the three Democrats have been ravaging each other in television ads, trading charges of incompetence, greed and mudslinging.
Checchi has been driving the ad war. He's spending more than $25 million of his own money and may set a national spending record for a non-presidential race by the time the election is over.
Checchi appeared defensive in the debate when asked why he had broken a promise to avoid negative campaigning.
Reading from a script, he cited "the guff I've gotten for a year now. ... I chose to respond on television."
He also said that what his rivals labeled as attacks, he would call "factual, not personal."
Despite Harman's condemnation of negative campaigning, she used her closing statement to slam her opponents.
She said Lungren was so "anti-choice, pro-gun and pro-tobacco" that he would be out of California's mainstream, Davis wasn't a "bold enough leader," and Checchi was "trying for a corporate takeover" of California.
"I say our state is not for sale," she said.
Lungren, who spoke last, countered: "Jane, thank you for that positive roundup on the rest of us," drawing laughter from the 300 people in the audience.
All four candidates are running in the open primary on June 2. The top two finishers from each party will face each other in November.
The latest poll pegged Davis as the Democratic front-runner with the support of 28 percent of likely voters , up from 12 percent last month Checchi with 19 percent and Ms. Harman with 8 percent. Lungren was favored by 23 percent.
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