Whitman, at a news conference an hour earlier, insisted she and her husband never saw such a letter. She said Santillan sorted their mail and "might have been on the lookout for that letter."
The letter, shown to reporters Thursday afternoon, included a handwritten notation that Allred said was written by Whitman's husband, Dr. Griffith Harsh.
"Nicky Please check this Thanks," is scribbled at the bottom of the letter addressed to Whitman and her husband.
Santillan kept the letter after Harsh gave it to her and the information requested by the Social Security Administration was not provided, Allred said.
"Now that we've shown you the evidence, let's see if she's going to deny it,"Allred said.
Whitman, at a news conference a few miles away and an hour earlier, said she would take a polygraph test to prove she did not know her housekeeper was undocumented "If it comes to that."
"I would be delighted to do that," Whitman said.
The allegations became public Wednesday when Allred held a news conference with Santillan to say the former housekeeper was "exploited, disrespected, humiliated and emotionally and financially abused" by the former eBay CEO.
"Make no mistake, these allegations are completely untrue they lack any merit whatsoever," Whitman said Thursday.
Whitman called the charges a "political smear" orchestrated by her Democratic opponent, Jerry Brown.
Allred said Thursday she has had no contact with the Brown campaign.
The Whitman campaign gave reporters copies of immigration and IRS forms it said Santillan signed stating she was a legal resident of the United States when she first applied for employment as a housekeeper in 2000.
