Christopher Lee Carlson, 41, entered his plea Tuesday in federal court in Portland.
The Oregonian reported that the letter to the Maryland Democrat's Baltimore office was part of a February 2012 mass mailing.
Sentencing for the man was set for Feb. 4. The newspaper said he faces a maximum penalty of five years, a $250,000 fine and restitution costs.
Carlson was indicted in March 2012 on 12 counts accusing him of sending the letters to six members of Congress, including Republican House Speaker John Boehner of Ohio.
The charges arose from an investigation into the mailing of about 100 envelopes containing white powder. The U.S. attorney's office in Portland said the letters, postmarked in Portland, tested negative for toxic substances.
A U.S. attorney's spokeswoman was not immediately reachable Tuesday night to discuss the status of the other counts.
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