(Bloomberg) -- Could Boris Johnson break the Brexit impasse by binding Northern Ireland -- rather than all of the U.K. as pushed by his predecessor -- closely to the European Union until another arrangement was reached?

On one hand, Johnson has ruled it out. On the other hand, he’s floated the idea of an all-Irish food zone as a way of avoiding checks along the frontier between Ireland, which is part of the EU, and Northern Ireland, which is part of the U.K. That suggests he might seek to revive a version of the Northern Irish backstop by another name.

Yet, it’s a path fraught with problems. We lay out three of the biggest ones below.

1. Veto Threat

Under current rules, a third of the members in Northern Ireland’s 90-strong power-sharing assembly can effectively block a measure they don’t like, using the so-called petition of concern. The mechanism has been described as an “open veto to be played like a joker at any time,” according to Mark Durkan, a former member of the assembly.

The fear in Dublin is that the Democratic Unionist Party could use the “joker” to stop London from drawing an effective customs border in the Irish Sea, separating all of Ireland from the U.K. In the most recent election, in 2017, the DUP took 28 seats -- but with the help of other unionist parties in the assembly, which is currently suspended, it could block rule alignment with the EU.

2. Scope

So far, Johnson has publicly focused on food as an area of potential alignment. Irish Prime Minister Leo Varadkar points out that this covers only about 30% of cross-border trade. That estimate might be on the low side. But, without rule alignment for other goods, that could mean checks are still needed around the border. In other words, what Johnson is offering is a stripped-down backstop and that’s not good enough, according to Irish officials.

3. The Numbers

A deal that involved some kind of separation between Northern Ireland and the rest of the U.K. might be a tough sell in parliament. This remains among the biggest concerns of Irish officials, who are now convinced Johnson wants a deal, but question his capacity to get an accord through parliament.

As things stand, he would need close to 50 Labour members of Parliament to vote for his deal. The Sun reported Monday that Tory whips believe they can persuade 20 more Conservatives to vote for a deal, meaning they would only require around 30 Labour MPs. That number might be achievable -- 26 signed wrote a letter in June calling for Brexit to be delivered.

Once again, the DUP move center stage. If they could find a way to support, or even abstain from voting against a Northern Irish backstop, it might unlock wider support for the measure. Certainly, the DUP doesn’t want a no-deal Brexit, and some very tentative signs of a softening have emerged. But, we’ve been here before, and in the end, the DUP has stood firm.

--With assistance from Robert Hutton.

To contact the reporter on this story: Dara Doyle in Dublin at ddoyle1@bloomberg.net

To contact the editors responsible for this story: Chad Thomas at cthomas16@bloomberg.net, Richard Bravo, Caroline Alexander

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