By James Cook
BBC News Correspondent, Rochdale
Mr Brown spent 40 minutes with Mrs Duffy trying to make amends
"But what do you really think?"
That question will dog Gordon Brown for the rest of the election campaign - perhaps for the rest of his political career.
This was the day the prime minister was caught facing two ways - expressing one view in public and quite a different one in private.
The picture of Gordon Brown listening as the private became very, very public on BBC Radio 2 said it all.
Mr Brown sat in front of the microphone, leaning heavily on an elbow, head in his hand, as he heard the recording of his own voice describing lifelong Labour voter Gillian Duffy as a "bigoted woman".
His body language spoke of utter dejection.
It was a disastrous day for the prime minister's election campaign - and in this instant he clearly knew it.
For the past three weeks Mr Brown's team have been desperate to talk about substance, accusing us - the media travelling with him - of unfairly focusing on style.
But the affair of Gillian Duffy has both substance and style.
First, the substance: immigration.
Oddly enough it wasn't actually Mrs Duffy's main complaint about the Britain she lives in today.
At first she wanted to focus on the deficit. I became aware of her discontent as I interviewed the prime minister about the economy, crime and cancer care. I could hear a voice in the background questioning where the money was going to come from.
And as Mr Brown moved away from our interview, Mrs Duffy moved in.
At the end of their lively conversation the prime minister appeared to have won her over.
We crowded around Mrs Duffy to hear her say that she didn't think he had answered all her questions but she was still voting Labour. It was probably as good an outcome as the Mr Brown could have hoped for.
And it was fairly typical of his engagement with "real people" on the campaign trail.
He generally treats voters with courtesy, listens to their concerns, asks and answers questions and appears to be on top of all the major issues.
In fact, face to face he is actually much better at serious political discussion than he is given credit for.
The PM's appearance drew a huge crowd outside Mrs Duffy's house
Most of the voters we have spoken to have come away from an encounter with the prime minister saying that they felt he was courteous, knowledgeable and polite.
But secondly, the style: everyone must now be asking...but what did he really think about me?
Mrs Duffy now knows what Mr Brown really thinks about her and it has almost certainly cost Labour a single vote in a constituency where a single vote might actually make a difference.
Rochdale is so marginal that boundary changes have shifted it from a Liberal Democrat seat with a majority of 442 in 2005 to a notional Labour seat with the majority estimated at just 149 at this general election.
Inside the red pebble-dashed terraced house of Gillian Duffy, a postal vote with a cross beside Labour sits in an envelope.
Before she met the prime minister Mrs Duffy intended to vote Labour. After she met the prime minister Mrs Duffy was going to vote Labour.
But the last we heard from her - and this was before Mr Brown returned to offer a face-to-face apology - Mrs Duffy had decided not to vote at all.