The Illinois senator was pressed by reporters at a news conference about his relations with the 52-year-old real estate developer and fast-food magnate. Rezko is charged with buying political influence with campaign money and using it to launch a multimillion-dollar shakedown scheme aimed at companies hoping to build hospitals or invest state pension money.
Obama has not been accused of any wrongdoing in the case brought by federal prosecutors.
Questions about the relationship dogged Obama as he wound up campaigning on the eve of crucial primary votes in Texas and Ohio in his hard-fought contest with Hillary Rodham Clinton. Obama campaigned exclusively in Texas on Monday, a state polls suggested he was more likely to carry than Ohio.
Pressed, Obama said, "Tony Rezko was a friend and supporter of mine for many years. These charges are completely unrelated to me, and nobody disputes that."
"There's no dispute that he raised money for us, and there's no dispute that we've tried to get rid of it," he added.
Obama did allegedly receive a $10,000 contribution made by a Rezko associate that is mentioned in the indictment. But Obama's campaign has long since sent the money to charity.
So far, Obama has sent $150,000 in Rezko-related contributions to charity. The contributions were to Obama's Senate campaign and his earlier campaigns for the U.S. House and state Senate.
Obama also was questioned about a 2006 real estate deal in which he became involved with Rezko.
"I have been very open about what I called a boneheaded move," Obama replied.
In June 2006, the Obama family purchased a home for $1.65 million near the University of Chicago where Obama has taught law and his wife, Michelle, is a vice president at the university's medical center. The same day, Rezko's wife, Rita, bought a lot next door from the same sellers for $625,000. Obama later told the Chicago Sun-Times the sellers had required that both lots be sold simultaneously.
The Rezkos later sold the lot, but not before selling the Obamas a 10-foot-wide strip of land from the property for $104,500.
"It was a mistake to have been engaged with him at all in this or any other personal business dealing that would allow him, or anyone else, to believe that he had done me a favor," Obama said in a written statement earlier.
Rezko's chief defense counsel, Joseph J. Duffy, on Monday scoffed at any speculation he might call Obama as a defense witness.
Courting the large military presence in Texas, Obama pledged at a veterans center in San Antonio to work for "a seamless transition from active duty to civilian life" for veterans returning from Iraq and Afghanistan, including expansion of the Veterans Administration system.
Later, he spoke to students at a town-hall meeting near Dallas and planned to wrap up his Texas campaign with a late-night rally in Houston.
In San Antonio, Obama told reporters he was optimistic about Tuesday's primaries in Texas, Ohio, Rhode Island and Vermont, but wasn't counting Clinton out.
"Senator Clinton is running a tenacious campaign. She is a hardworking candidate who brings to bear enormous resources.
(Former) President Clinton is working hard on her behalf. We know this has been an extraordinary election. It continues to be.
"We're working as hard as we can to try to do well," Obama said.
Obama campaign manager David Plouffe called Tuesday "the last big window of opportunity" for Clinton, noting that "enormous leads" she enjoyed as recently as two weeks ago had dwindled or evaporated.
