Rebecca Long-Bailey has thrown down a challenge to her Labour opponents: calling on them to commit to maintaining Jeremy Corbyn’s commitments to nationalise mail, rail and energy.

It’s the second attempt that Long-Bailey has made to tie her opponents down on policy, the second being on the Green New Deal. Keir Starmer, who every indication we have suggests is still comfortably ahead in the Labour leadership contest, is largely running on his biography, and is leveraging the fact that he fits the stereotypical image of a British prime minister. Lisa Nandy, the best media performer of the trio, is fighting a tactically smart campaign in which she talks up the need for Labour to make “bold” choices to save itself – but is vague about what those bold moves are.

So it makes sense to try to force Long-Bailey’s two rivals to define themselves in ways that might lose them support among party members. The problem is while the strategy is sound, Long-Bailey is making a hash of it. Let’s take her original pitch in Tribune, which used her creation of Labour’s package of measures to tackle climate change as a dividing line. The problem is that Labour’s “Green New Deal” has already been watered down by the party’s trades unions. It’s true to say that the party management skills that Long-Bailey displayed in carrying that through are one essential skillset every successful Labour leader must possess, but the result of that is there is simply nothing in the actually existing Green New Deal that any member of the Parliamentary Labour Party would struggle to support, and nothing in it that places barriers in Labour’s path to power.

What makes a dividing line effective is if your opponent finds it painful to step over it. This isn’t it. Neither is the commitment to “mail, rail and energy”. These are all issues that are popular with the public, but crucially, they are low salience – which means that while voters support them, they aren’t all that concerned about it

The mistake Labour makes when it talks about its policies being popular is a lot like being confused when you’re turned down for a date at Five Guys because the person you asked out has previously told you they really like hamburgers. It’s not the meal choice that’s the problem – you can keep suggesting burger joints, but unless you change what the object of your affection actually dislikes, you are still getting dinner alone.

Starmer and Nandy might both privately think that they should junk many of those nationalisation commitments because they would rather focus the attention of voters elsewhere, or because they think that the political benefits of running them are exceeded by the costs of making them work well when they could do other things, or because they are ideologically or practically opposed to some or all of them. But bluntly there is no cost at all to either of them in saying they back these commitments, because the public is not opposed to them, but if they junk them later, they just won’t care.

In any case, Starmer has already talked of the 2017 manifesto as “foundational” and has supported the principle of public ownership. These are not commitments that Long-Bailey’s rivals struggle to match. The big hint here is that Tony Blair promised to renationalise the railways in his 1994 Labour leadership campaign. Frankly if you are the Corbynite candidate and your dividing line is one Tony Blair could step over, what are you doing?

It’s true to say that Nandy has said in the past that she would not back nationalising the energy companies and would prefer instead to set up co-operative alternatives, but bluntly, Nandy has already shown that she is willing to U-turn on key positions to stay in contention in this leadership race. My general view is that her supporters in the membership know at a subconscious level that they are being offered a substantially different version of the pitch she will adopt as leader – but they see that she is the most charismatic candidate in the field and want an excuse to overlook it, just as Labour members in 2015 were happy to accept Corbyn’s line that his mind was not “closed” to supporting Brexit because they wanted to vote for the most committed opponent of austerity in the field.

Long-Bailey and her campaign seem to be swallowing their own propaganda, both about the depth of the radicalism of the Green New Deal as it actually existed as party policy, and about the tactical stupidity of their opponents. If she wants to find a way to unpick her opponents’ coalition, she will likely need a better, and more fruitful set of dividing lines.