Imagine the scene. Theresa May’s government faces a critical Brexit vote – this spring, perhaps – on whether Britain should remain in a customs union; or in October on whatever divorce deal it has managed to negotiate with the European Union. With the support of the Democratic Unionist Party (DUP), and through ferocious pressure from Conservative whips on the party’s Europhile backbenchers, the government believes it has just enough votes to carry the day.
Sinn Fein then makes its dramatic move. It announces that for this vote, and this vote only, it is abandoning its century-old policy of abstentionism. After an emergency Ard Fheis (party conference) its seven MPs come to Westminster, swear allegiance to the Queen through gritted teeth, and ensure the government’s defeat.