British Prime Minister Rishi Sunak’s government is beginning its hardest sell: convincing skeptical Northern Irish politicians to back its new post-Brexit deal on trade with the European Union. That means one final effort before the U.K. can finally repair relations with its biggest trading partner.

Monday’s unveiling of the Windsor Framework featured a confident Sunak and a smiling, enthusiastic European Commission President Ursula von der Leyen. The next, crucial stage of the process will be a game of detail, numbers and political arm-twisting.

Sunak’s team now needs to win over a tiny caucus of Northern Ireland’s unionist politicians and an influential group of pro-Brexit MPs from his own Conservative Party. Failure could mean a need to push the deal through with support from the opposition Labour Party — a move that could be politically damaging for Sunak.