He rejected long-standing allegations that Blair misled parliament when he claimed the Iraqi leader had weapons of mass destruction (WMD), and accused the media of "settling your scores" in their coverage of the issue.
Campbell became emotional and had to pause to compose himself at one point, saying: "I'm sorry if I do get upset about this but I was there alongside Tony, I know how that decision weighed on him, I know the care that we took."
Blair's appearance on January 29 was the highlight of the ongoing inquiry into the war, which remains highly divisive here despite Britain's formal withdrawal from Iraq in July.
His remark at the hearing that he had no regrets in removing Saddam, even though no WMD were found, sparked outrage among relatives of the 179 British soldiers who died in the conflict, as well as in many sections of the media.
The issue is likely to be damaging for Blair's Labour party and the man who took over from him as prime minister, Gordon Brown, ahead of a general election which must be held by June.
Brown was finance minister during the invasion and is due to make his inquiry appearance before the vote, which Labour is widely tipped to lose.
The Iraq war dogged the second half of Blair's time in office and contributed to his decision to resign in 2007, but he has consistently rejected criticism of his motives over the past seven years.
Questioned about why Blair said it was "beyond doubt" that Iraq had WMD when the intelligence was patchy at best, Campbell was equally defiant, saying: "The prime minister did not mislead parliament."
Pressed further, however, he paused and became emotional.
"I've been through a lot on this," he said. Composing himself, he continued: "Tony Blair, I think, is a totally honourable man.
"And I also think that what we've taken on this... this constant vilification... I think the way that this whole issue has developed now is, I don't think people are interested in the truth.
"I think you're all (in the media) interested in settling your scores and setting your own agenda."
Later Sunday, a fresh row between Campbell and the BBC -- who locked horns repeatedly over the broadcaster's coverage of the war -- appeared to be brewing over the way the interview was conducted.
Writing in his blog, he said the interviewer's bias was "exposed in the way he casually threw in a highly disputed figure about casualties -- four to five times higher than the Iraq body count accepted by most organisations as the most reliable."
The BBC journalist questioning the former communications chief put the death toll following the war at 600,000, saying the data came from the United Nations.
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