SANTA MONICA, Calif. — Meg Whitman's campaign for governor was thrown into turmoil Thursday as the Republican sought to fend off new evidence that she knowingly had an illegal immigrant housekeeper on her payroll for nearly a decade. Whitman denounced the allegations as a "baseless smear attack" by Democratic challenger Jerry Brown in what has become a dead-heat race five weeks before the election. The central issue is whether Whitman knew about a letter that the Social Security Administration sent her in 2003 that raised discrepancies about the housekeeper's documents — a possible tip-off that she could be illegal. The letter is the foundation for claims by former maid Nicky Diaz Santillan that Whitman and her husband knew for years she was in the U.S. illegally, but kept her on the job regardless. For two days, Whitman forcefully denied receiving any such letter and said she fired the $23-an-hour housekeeper last year immediately after learning she was illegal. But Whitman's husband changed course Thursday after a letter surfaced with what appeared to be his handwriting, forcing him to say he may have been aware of the correspondence back in 2003. The husband's shift only served to intensify the uproar in a contest that until now been focused on serious issues such as job creation, government spending and education in a state with a $19 billion deficit and 12.4 percent unemployment. Now, the focus is on whether the billionaire GOP nominee for governor will take a polygraph test to respond to allegations brought by a celebrity-seeking attorney and her mysterious housekeeper client. Revelations about the illegal housekeeper have also thrown Whitman's carefully managed campaign completely off track and opened the door for Democrats to accuse her of hypocrisy. The former eBay chief executive has called for tougher sanctions against employers who hire illegal workers, and the fact that she employed an illegal immigrant maid from Mexico.