Brexit: DUP chief Foster has power but will have to handle it with care: writes Suzanne Breen
Seldom have such a small number of voters held such massive influence. The 292,000 voters who supported the DUP in June's Westminster election appear to have vetoed an agreement affecting more than 500 million people in the EU.
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The events which yesterday led to Theresa May withdrawing the deal she'd agreed with Brussels shows the enormous, unprecedented power that Arlene Foster's party currently has over the government.
Sinn Fein deputy leader Mary Lou McDonald has predicted that the DUP-Tory confidence and supply deal will end in tears. She may well be right, but last night the unionist party was celebrating its victory.
Although not too publicly. There were no outbursts on social media from even those normally ebullient MPs. They're on a good thing and they have no desire to rub Mrs May's nose in it or irritate the wider British public.
It was all going well for the Prime Minister and the Taoiseach yesterday.
An agreement was being struck to solve the problem of the Border and move onto the next round of the Brexit negotiations. Northern Ireland would apparently still be bound by EU rules.
Then Mrs Foster gave a press conference at Stormont stressing her party would "not accept any form of regulatory divergence" separating Northern Ireland "economically or politically from the rest of the UK".
A telephone call to Mrs May followed, and it was game off in Brussels. The Prime Minister was left looking red-faced and ridiculously weak.
But yesterday raised as many questions as it answered. It's almost incredible to believe that the Tory leadership didn't run the text of their proposed deal past the DUP.
The parties are in contact not just on a day-to-day but often hour-to-hour basis.
The closeness of the relationship between de facto deputy Prime Minister Damian Green and Nigel Dodds and his team of MPs was clear for all to see at the DUP conference dinner just a week ago.
The DUP makes no secret of its position on Brexit regarding Northern Ireland.
Mrs May should surely have known that the party would pull the rug from under her if she moved against its wishes.
Either she was trying to bounce them into agreeing, or she just didn't take them seriously enough. But senior party sources have repeatedly told this newspaper that they wouldn't hesitate to bring down the government if they believed Northern Ireland's constitutional status was being threatened.
While an election could mean a Corbyn government and an end of influence for the DUP, its electoral position in Northern Ireland would be unaffected, as it wouldn't lose a single seat and would likely even take an extra one, North Down. The Tories, by comparison, would face serious losses.
Yet the DUP has reason for caution too.
If the Prime Minister was prepared to do the deal despite previous reassurances to Mrs Foster's party, then it's a case of Perfidious Albion - and it could well be again.