British Prime Minister Rishi Sunak is gearing up for the biggest political test of his premiership as he prepares to unveil a deal with the European Union that risks a stand-off with Northern Ireland unionists and members of his own party.

A solution to the impasse over the post-Brexit trading rules for Northern Ireland would enable the U.K. to reset relations with the EU, its biggest trading partner, more than the three years after Britain formally left the bloc.

The prime minister also hopes to persuade the Democratic Unionist Party to drop its veto on the formation of Northern Ireland’s power-sharing executive. They’ve blocked its functioning for more than a year in protest at the so-called protocol, the portion of the Brexit deal governing the region’s unique place in the EU and U.K.’s trading markets.