DUP’s Nigel Dodds and Arlene Foster react after news of the deal PA

Boris Johnson beaming from ear-to-ear amid bonhomie and backslapping with EU leaders in Brussels.

The mood was very different as DUP politicians gathered closer to home. Arlene Foster and Nigel Dodds had faces like thunder as they faced the media after BoJo's betrayal.

We can go back to the DUP annual conference last year when he was the star turn, mesmerising delegates with a sizzling speech.

Or four months ago when he and Arlene stood regally on that Stormont balcony and it looked like they would rule the realm in harmony for years to come.

But we don't need to go back even that far to measure the scale of Tory treachery.

Just a fortnight ago Boris bedazzled at a DUP drinks reception at the Conservative Party conference.

"Boris! Boris! Boris!" shouted the assembled Duppers. The Prime Minister could barely get a few sentences in succession past his lips because he was so regularly interrupted by cheers.

The DUP will try to explain away this U-turn as the Prime Minister being under immense pressure from Brussels and Dublin, but that alone won't wash.

The party really needs to consider its own conduct over Brexit.

It has revelled in its high-profile status, and now it's paying the price. It looks stupid on a national level.

All those Kodak moments of walking into Downing Street for talks, tete-a-tetes with Government VIPs, and endless meetings. For what? Nothing was achieved. This deal arguably poses a far greater danger to the Union than Theresa May's ever did.

None of the DUP's confident Brexit predictions have come to pass. Brussels didn't dump Dublin; Leo has paid no price for playing hardball; Angela Merkel never rode to the rescue.

Maybe the next time the DUP hears a Tory Prime Minister laud the "precious Union", it will treat that pledge with a healthy scepticism.

Ironically, the party must pin its hopes on a unionist-hated Labour leader managing to successfully whip his MPs for tomorrow's Commons vote.

It certainly can't count on its old ERG friends, who were last night making positive noises about Boris' deal. The numbers are so tight that everything could well hinge on a handful of votes.

If those 10 DUP MPs stop the deal passing, then their party will save face after placing so much faith in the Tories.

But parliamentary defeat alone won't spell the end of these proposals. Don't expect Boris to fight a general election threatening no-deal. He'll campaign on his deal and if he wins a majority - as the opinion polls all suggest - the DUP will have to watch from the sidelines as it becomes law.

"Saturday's vote in Parliament on the proposals will only be the start of a long process to get any Withdrawal Agreement Bill through the House of Commons," the DUP has said.

The party is right - the game's still on, the final whistle hasn't been blown. But unionists will now have to come back from behind if there's even to be a score draw.

Belfast Telegraph