Weeks before his death, the late state Treasurer Jesse M. Unruh, one of the most accomplished political fund-raisers in California history, wrote a letter endorsing a ballot proposition to reform and restrict campaign financing in legislative races, according to sponsors of the measure. Officials of Common Cause and the League of Women Voters released the letter at a press conference in Los Angeles. "I don't think anybody has more credibility on the issue of money and politics than Jesse Unruh," said Common Cause's Walter Zelman. Backers of the proposition, which will be on the ballot in June, said they hoped release of the 5-month-old letter would spur trustees of Unruh's estate to contribute some of his remaining $1.3-million political war chest to the campaign. Unruh, a Democrat and former Speaker of the Assembly, was famous for the saying: "Money is the mother's milk of politics." But Zelman said Unruh concluded before his death on Aug. 4 that "the milk has gone sour."