SACRAMENTO, Calif. (AP) — Supporters of Gov. Jerry Brown's November tax initiative, including both of California's U.S. senators, called Thursday on a rival campaign to stop its criticism, warning that attacks and negative campaigning will jeopardize prospects for both measures.

School funding would be hardest hit if voters are turned off and reject the initiatives, Brown's backers said in their letter to the California PTA.

It said the campaign for Proposition 38, funded by wealthy civil rights attorney Molly Munger and backed by the PTA, has become increasingly negative in recent months, even personally attacking Brown.

"These attacks have called the governor 'untruthful,' attacked Prop 30 as a 'shell game,' and a 'tiny Band-Aid' that is 'terrible for kids,' and even included the false claim that Prop 30 will make schools lose money," the letter states.

It is signed by Sens. Dianne Feinstein and Barbara Boxer, Senate President Pro Tem Darrell Steinberg and Assembly Speaker John Perez, all Democrats.

The letter proposes that both campaigns adopt a positive campaign compact in which they refrain from directly attacking or referring to one another in all ads, public statements and voter outreach.

"While we may not all agree on the ultimate funding solution, education advocates across California can surely agree on the worst outcome for our state: one where both initiatives fail," it said.

The letter was addressed to PTA President Carol Kocivar. She said she had just received a copy Thursday afternoon and wanted to craft a thoughtful response before commenting.

Nathan Ballard, a spokesman for the Proposition 38 campaign, said Kocivar has a great deal of respect for the senators and would respond to them directly.

"In the meantime, we will continue to campaign vigorously for Proposition 38," Ballard said in an e-mailed statement. "Prop 38 makes a bigger investment in California's public schools than any other measure, and we're going to continue to make that point, in a respectful and courteous manner, to the voters of this state."

Brown, a Democrat, officially launched his campaign for Proposition 30 this week at a Sacramento high school. Afterward, Ballard called the news conference "stagecraft," and said the governor's initiative would not provide as much money for schools as Proposition 38.

An email he sent to reporters compared the amount of money New Technology High School, where Brown appeared, would receive from each initiative ($145,000 under Brown's initiative and $336,000 under Munger's).

Brown's initiative proposes raising income tax rates on a sliding scale on incomes over $250,000 for seven years and increasing the state sales tax by a quarter cent for four years to help ease California's budget woes. If voters reject it, the Legislature could impose about $6 billion in automatic cuts, primarily to schools and higher education.

The initiative backed by Munger, the daughter of billionaire Berkshire Hathaway Inc. Vice Chairman Charles Munger, would raise income taxes on a sliding scale on nearly all income levels for 12 years and send most of the money directly to school districts on a per-pupil basis. She has contributed nearly $19 million of her own money to back it.