In late August, state Sen. Tom Harman (R-Costa Mesa) missed a vote on a bill that would stiffen penalties against people who defraud senior citizens and tighten banking regulations.

Harman's absence had no bearing on the outcome, as Senate Democrats easily approved Senate Bill 586 in a 25-13 vote, with Republicans almost unanimous in their opposition.

But it was what Harman did next — sending Gov. Jerry Brown a letter Sept. 8 urging him to veto the bill — that left the bill's biggest proponent puzzled.

"It just struck me as odd, when I found out that he represented such a large contingency of seniors, why you would do that?" said Liz Sanders, a Woodland Hills resident whose mother was victimized by her caretaker and has made this bill her solo pursuit for almost a year. "I'm thinking, 'You're up there in age; you're not that far up from this benefiting you.'"

In 2009, Sanders' mother, Bette Isenberg, was bedridden, suffering from a multitude of health complications. Unable to go to the bank or sign her name, Isenberg's caretaker used a signature stamp to handle Isenberg's finances.

Within a year, that caretaker would be sentenced to more than two years in prison for grand theft.

The woman had stolen more than $750,000 of Isenberg's money, in one instance opening up a $300,000 bank account thanks to the signature stamp and a letter to the bank.

"Just with a stamp," Sanders said. "A stamp."

S.B. 586, authored by Senator Fran Pavley (D-Agoura Hills), would double the fines for defrauding seniors, regulate how California banking institutions issue signature stamps, and funnel money from convictions to county adult protective services.

The bill is sponsored by the American Assn. of Retired Persons (AARP), the County Welfare Directors Assn. and the Los Angeles district attorney's office, among others.

"These stakeholders could neither indicate the frequency with which signature stamps are used by account-holders nor the frequently (sic) with which signature stamps are used to perpetrate fraud or to steal from account-holders," Harman wrote in his Sept. 8 letter to Brown. "The Legislature, in essence, is 'protecting' account-holders from a problem which may not even exist!"

The bill is opposed by the California Bankers Assn. — a donor to Harman's most recent campaign — and the California Independent Bankers.

"Signature stamps are not widely used today, and to the extent that they are, financial institutions are aware of very few fraud problems," Harman wrote.

California has more than 4.2 million senior citizens, according to 2010 U.S. Census data. There are more than 110,000 seniors living within Harman's district, and another 150,000 between ages 50 and 64.

"Sen. Harman is sympathetic to the problem S.B. 586 tries to address, but didn't see any evidence that it was widespread," Harman's communications director, Eileen Ricker, wrote in an email. "It just seemed premature to pass laws and restrictions when there does not appear to be a pervasive problem."

Disability Rights California, an advocacy group for the disabled, estimated that 1% of the state's 30,000 disabled persons use signature stamps.

Critics argue the new funding for county agencies could create temporary complications in imposing criminal sentences, and argue it's unfair to limit the law to only California banks instead of national chains like Wells Fargo and Chase.

Brown has until Oct. 9 to sign the bill into law.

joseph.serna@latimes.com

Twitter: @JosephSerna