While an Illinois state senator, Obama was key in getting the state's notorious death penalty laws changed, including a requirement that in most cases police interrogations involving capital crimes must be recorded.
The changes enacted in 2003 reformed a system that had sent 13 people to death row, only to have them released because they were later determined to be innocent or had been convicted using improper methods.
"Without Barack's energy, imagination and commitment I do not believe the very substantial and meaningful reforms that became law in Illinois would have taken place," said author Scott Turow, a member of the state commission that recommended many of the changes.
Obama often cites his role in Illinois death penalty debate as evidence that he can resolve thorny issues through compromise.
"We brought police officers and civil rights advocates together to reform a death penalty system that had sent 13 innocent men to death row," he declare in a recent presidential debate among candidates in the 2008 election.
Enactment of the 2003 law was a huge political achievement in a state that had been deeply divided over problems with capital punishment.
Obama was at the center of the emotional debate.
Legislators and lobbyists who worked with him describe a lawmaker who was personally involved, refused to abandon some needed changes but also demanded compromises from both law enforcement and death penalty critics.
A proposal to require that police record interrogations of murder suspects was opposed by police, prosecutors and the Democratic governor and considered so touchy it was separated from other legislation. It also was the issue that garnered Obama's special interest.
"I thought the prosecutors and law enforcement would kill it," said Peter Baroni, who was then a Republican aide to the Illinois Senate's judiciary committee. "He (Obama) was the one who kept people at the table."
In the end, police organizations supported the recording mandate, and the measure passed the Senate unanimously.
Illinois ' death penalty was an emotional issue in 2003. The courts had released 13 people from death row because evidence had turned up proving their innocence or that their convictions had been tainted.
The previous governor, Republican George Ryan, had halted all executions and commuted the sentences of everyone awaiting execution, giving most of them life in prison.
The families of many murder victims felt betrayed. Police and prosecutors felt their every move was being criticized. Death penalty foes were jubilant but also divided over whether to push for an outright ban.
Lawmakers were looking for way to solve the problems in the law, but also worried being labeled "soft on crime."
For Obama, a student of constitutional law, it was an issue he relished to tackle _ and also one of keen importance to the black voters he would need if he ran for the U.S. Senate in 2004.
The idea that people might be executed for crimes they did not commit also enraged him. "At minimum, we should agree that innocent people should not be put to death by the state. At minimum," Obama declared icily during one floor debate.
Obama saw the issue of police interrogations as key.
Among the men released from death row "a consistent pattern was the faulty confession," argued Obama. "It struck me that this was the hardest piece of the puzzle but the one that would ultimately make the most difference and have the most long-lasting effect."
Participants in the negotiations describe Obama as standing firm on some issues, but willing to compromise on others.
They cite his refusal to narrow the law so that only a suspect's confession had to be recorded, insisting that the entire interrogation be put on tape, so a suspect cannot be threatened or beaten off camera.
"That was a first point at which he could have taken the easy route. He said no, we're not doing it that way," recalled Kathryn Saltmarsh, who represented the Illinois Appellate Defender's office in the negotiations.
On other things he was willing to compromise.
He went along with allowing departments to make audio recordings if they could not afford video equipment and training, and for a judge to allow an unrecorded statement in some cases - but then prosecutors would have to prove it had been obtained without coercion.
These exceptions were critical to winning the support of law enforcement, said Laimutis "Limey" Nargelenas, who represented the Illinois Association of Chiefs of Police in the discussions.
Obama "could have rammed (the legislation) through, but he was willing to work with us," recalls Nargelenas.
"He is just really a good legislator," says State Sen. John Cullerton, a Democrat who oversaw the broad package of reforms including raising standards for death sentences and making it easier for judges to overturn unfair sentences.
"I don't know if that will get you many votes for president, but he was an excellent negotiator."