"If you're innocent and somebody charges you, would you give up? No, you've got to fight," said Tan Nguyen, who is seeking to unseat five-term Democratic Rep. Loretta Sanchez in California's 47th Congressional District.
"Innocent people can be persecuted," he told The Associated Press.
The state Department of Justice has opened a voting rights investigation into the letter sent to certain Democratic voters in Orange County.
The letter, written in Spanish, warned: "You are advised that if your residence in this country is illegal or you are an immigrant, voting in a federal election is a crime that could result in jail time."
Immigrants who have become naturalized U.S. citizens are eligible to vote.
Nguyen, a Vietnamese immigrant who has made illegal immigration a centerpiece of his campaign, was interviewed at his campaign office. It was open for the first time since being raided last week by investigators who seized computers, political signs and other items.
He became visibly angry while talking about the search. His face turned red and he pounded a fist on his desk as he spoke.
Nguyen said he purchased a database of 14,000 Hispanic voters so he could send Spanish-language fliers to voters. He said his campaign sent four fliers using the database before his office manager forwarded the names to an outside party.
Nguyen refused to identify the office manager or the third party whom he said mailed the letters.
He reiterated that he did not authorize or approve the intimidating letters and said neither he nor any of his three paid campaign staff members wrote or funded the mailing.
"They didn't write it. I didn't write it," he said.
Asked if any campaign volunteers were responsible for the mailing, Nguyen referred questions to his attorney.
Nathan Barankin, spokesman for the state Department of Justice, had no immediate comment on Nguyen's account. Sanchez's campaign has said it would not comment until after the investigation is complete.