Smith, from St. Louis, pleaded guilty last month to two federal counts of obstruction of justice and resigned his state office. The St. Louis Post-Dispatch on Tuesday published a letter from Smith, who apologized to fellow senators, family members and friends and those who lost faith in government.

"Telling the truth is the basis of public trust: the minimum I owed my constituents, my family and myself," Smith wrote. "I am a reminder of the obligation to always be truthful, particularly for those honored to serve the public."

Smith narrowly lost to Carnahan in a crowded 2004 primary to replace retiring U.S. Rep. Dick Gephardt. Smith, 35, was a political science instructor at Washington University in St. Louis and made his political debut in the race. He was elected to the state Senate in 2006, and left office when he pleaded guilty.

Smith's legal troubles began when a political operative approached his congressional campaign in July 2004 and offered to produce negative ads aimed at Carnahan. Smith approved a plan to distribute negative campaign material against Carnahan using a supposedly independent campaign committee.

Carnahan's campaign filed a complaint with the Federal Election Commission alleging improper coordination between Smith's campaign and the supposedly independent group that distributed the postcards and fliers. Smith then submitted a false affidavit to federal investigators claiming no involvement in the attack.

Smith in the letter published Tuesday called himself "fiercely competitive" and explained that he was seeking any advantage against Carnahan, who still holds the congressional seat. Smith said he lied to the FEC to preserve his political future and because he presumed investigators would not "connect the dots."

Smith also said he feared having the scheme be discovered because of the embarrassment it would cause his parents to read about the details in the newspaper and the shame they would feel.

"I knew that hiding my campaign's involvement was against the law. I was raised better than that, but I thought the ends justified the means," Smith wrote.

The FEC decided in December 2007 not to take any action against Smith, saying there was no reason to believe federal laws were violated. The FBI and U.S. attorney's office in St. Louis opened a new investigation in January into Smith's congressional campaign.

Two campaign aides for Smith's campaign - Nick Adams and Steve Brown, a fellow St. Louis-area Democrat who was elected to the House in 2008 - also pleaded guilty to charges related to obstruction of justice.

Smith, Brown and Adams face a maximum of 20 years in prison and a $250,000 fine on each count, but court documents show sentencing guidelines of 15 to 21 months. Prosecutors have said they would recommend reducing sentences for Brown and Adams for their cooperation.

Sentencing for all three is scheduled for Nov. 10. A special election to replace Smith and Brown in the Legislature is scheduled for Nov. 3.