Billionaire Meg Whitman has contributed an additional $13 million of her own money to her effort to become California’s next governor, bringing the Republican nominee’s personal stake in her election bid to $104 million.

The donation was filed Friday with the California secretary of state’s office. Whitman, the former chief executive of EBay, is in a dead heat with Democratic nominee Jerry Brown despite the fact that she has vastly outspent the California attorney general since she won the primary election in June.

Brown has said his shoestring campaign is a necessity during the summer, and he is marshaling his resources for the fall. But his campaign has been the beneficiary of independent expenditures from unions, which the Whitman campaign said is what prompted her to write another large check.

“Meg is investing the necessary resources to fight the $13 million in attacks Jerry Brown Inc. has launched to defend the status quo in Sacramento,” said spokeswoman Sarah Pompei.

Brown’s campaign painted the move as an act of desperation by a candidate whose record spending has failed to produce results in the polls, and mocked Whitman for filing the notice on a lazy summer Friday, when it is unlikely to receive much attention.

“Meg Whitman continues to pour unprecedented millions into a campaign of unprecedented dishonesty,” said spokesman Sterling Clifford. “Her ads have been proven false, her trickle-down tax breaks for the super-rich are a proven failure and the late-night disclosure is all about keeping voters in the dark.”

seema.mehta@latimes.com