CHICAGO (CBS) — An ethics investigation centering on U.S. Rep. Jesse Jackson Jr. won’t be going away anytime soon.

Congressional investigators have revealed for the first time that they have more questions about whether Jackson tried to buy President Barack Obama’s old Senate seat.

The Office of Congressional Ethics now says they also believe Jackson improperly used his Washington and Chicago staffs to mount a “public campaign” to secure that appointment.

Jackson maintains his innocence, saying in a statement: “The Ethics Committee will continue their legal responsibility and I will continue to fully cooperate with them. At the end of the process I still believe I will be vindicated.”

Jackson his accused of directing staffers in 2008 to raise money for former Gov. Rod Blagojevich. In return, Blagojevich would appoint Jackson to President Obama’s old Senate seat. Blagojevich is scheduled to be sentenced for a variety of corruption convictions, including efforts to trade the Obama appointment.

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U.S. Sen. Dick Durbin says Jackson deserves the benefit of the doubt.

“I think we owe it to him and to the cause of justice not to jump to conclusions. Let the House Ethics Committee do their job,” he said.

But the senator stopped short of endorsing Jackson.

“I’m not going to make any endorsement in that race,” he said. “I think you can understand that.”

Congressional opponent Debbie Halvorson says the seven-term congressman is clearly distracted.

“The first thing I thought of was this could go on for years,” Halvorson said. “And here we are, just another day where constituents aren’t getting taken care of.”