"I agree with Diane"

There is a token candidate for the Labour Party leadership who struggled to get enough nominations to stand and who has no chance of winning.

His name is "Andy Burnham".

Ed Balls has more support amongst MPs, but he can't win either, with his 61% disapproval ratings.

Diane Abbott, in contrast, is a serious contender. She occupies the centre ground in policy terms - anti-Iraq war, anti-NHS privatisation, pro-equality and in favour of reducing the deficit by taxing the rich rather than cutting public services. Even before she gets a bounce from all the favourable media coverage, polls show her as the top choice amongst the general public, and she could easily end up topping the poll amongst Labour members and trade unionists, who between them have two-thirds of the votes in Labour's electoral college. It's also difficult for her opponents to criticise her when they will want the second preferences of her supporters.

She's got weaknesses - no ministerial experience, very little support amongst MPs and not much of a campaign team or organisation. She could easily make a gaffe or end up running a ridiculous campaign and end up as a bit of a joke. But she's arguably starting from a stronger position than Harriet Harman had at close of nominations for the Deputy Leadership.

One of the things which we learned from the first leadership debate back in April was that the leaders of the Labour and Tory parties had absolutely no idea how to respond to an articulate populist politician making centre-left arguments. It will be interesting to see how the Miliband brothers and the fringe right-wing candidates, all of whom have ministerial records to defend, respond to Abbott. My guess is that we will hear a lot of "I agree with Diane".

And let's remember the lessons from Labour's deputy leadership election. Jon Cruddas won all the policy arguments just by making some reality-based soft left arguments, and Harriet Harman got elected with the argument of "I agree with Jon + I've got experience of being in government + I'm a woman'. We saw then that there was a clear gap between the opinion of most Labour MPs and the members and trade unionists. Many MPs and Westminster insiders, for example, thought that Hazel Blears was a strong candidate with mainstream views, who connected well with "ordinary people". Amongst the wider party, Hazel Blears was a joke candidate who supported marginal and ridiculous policies and got a derisory result. I can't imagine that gap between MPs and the wider party has closed much over the past three years.

Abbott's challenge can only be a good thing for the Labour Party. She is an articulate and populist candidate who will put forward leftie arguments which the people in charge of the party have ducked out of debating for all these years. We'll be able to find out which leftie policy ideas capture the public's support and which ones belong in the dustbin of history. Her candidacy will persuade more people to join up and get involved. It kills off the movement to try and make Labour the anti-immigrant party. And whoever emerges as Labour's next leader will have sharpened and developed their campaigning skills, and be all the better prepared to help Labour win the next election.