Sir Jeffrey Donaldson has promised to use his leadership of the Democratic Unionist party (DUP) to campaign against the Northern Ireland protocol. The Lagan Valley MP aimed the volley at the British government and European Union on Tuesday after becoming the only candidate to succeed Edwin Poots, who resigned as DUP leader last week. Donaldson, who is to be confirmed as leader on Saturday, said his first priority would be to lobby Boris Johnson about the Irish Sea border, which many unionists view as a threat to British identity. “I will be speaking with the prime minister at the earliest opportunity to emphasise that it is not realistic to expect stability when every unionist representative in the devolved institutions opposes the Northern Ireland protocol,” he said. “The government and those who claim to be protectors of peace and stability, must step up and deal with the protocol in a manner which respects the constitutional and economic integrity of the United Kingdom. I will play my part but the government and Brussels must step up and recognise the flaws of the protocol and how it was foisted upon Northern Ireland.” The DUP hopes Donaldson’s coronation will stabilise the party after months of turmoil. Poots led an internal revolt that ousted Arlene Foster in April and narrowly beat Donaldson in a two-horse race last month, only to be ousted himself in another revolt last week, leaving the fate of his protege, the first minister, Paul Givan, unclear. There have been fresh suggestions that the protocol will be diluted, despite the insistence of the European Commission it must be adhered to. Poots said the Northern Ireland secretary, Brandon Lewis, had assured him of “very significant” changes. “The UK government are not going to tolerate how things are,” he told the BBC. “We are looking to these changes happening in July, most likely early July.” David Frost, the Brexit minister, said the EU needed to show more pragmatism and that it was “hard to see” how the protocol could survive in its current form. The Irish Sea border – and efforts to remove or blur it – is expected to dominate Donaldson’s tenure. The DUP chairman, Lord Morrow, said he had received one valid nomination for the leadership. “Following the completion of our party processes, Sir Jeffrey Donaldson MP will be the next leader of the Democratic Unionist party,” he said. The party’s electoral college is expected to confirm the decision on Saturday, with the party’s executive ratifying it next week. The 58-year-old father of two has eyed unionism’s crown during a long political journey. Born into a big Presbyterian family in Kilkeel, a fishing village on the coast of County Down, Donaldson played the Artful Dodger in a production of the musical Oliver! at school. He served in the Ulster Defence Regiment, was a deputy grand master in the Orange Order and started his political career in the Ulster Unionist party (UUP) as an aide to Enoch Powell. He later defected to the DUP to protest at Good Friday agreement-era concessions to Sinn Féin. After Ian Paisley led the DUP into power-sharing with Sinn Féin, Donaldson found a niche as the party’s star media performer, varnishing prickly positions into smooth BBC and RTÉ interviews. The move against Foster in April – largely fuelled by anger at the protocol – left him on the back foot. Donaldson was associated with Foster and the discredited DUP team that let Boris Johnson clinch a Brexit deal that led to an Irish Sea border. In a vote among DUP Stormont assembly members Donaldson lost to Poots by 17 votes to 19. Poots’ disastrous 21 days as leader ended in a party revolt over a deal on Irish language legislation, prompting the party to turn to Donaldson. He will need all the Artful Dodger’s nimbleness to save the DUP. Riven by infighting and defections, polls show the party shedding support to the hardline Traditional Unionist Voice, a revived Ulster Unionist party (UUP) and Alliance, which appeals to unionists fed up with constitutional arguments. “The DUP electorally is far more likely to do better under Donaldson than it could ever have managed under Poots,” said Jonathan Tonge, a University of Liverpool politics professor who is an authority on the DUP. “I don’t think it will ever revisit the electoral heights it reached in 2016-17 under Foster.” The new leader prefers soft tones to traditional Paisleyite thunder but he will need to show flintiness over the protocol, said one party insider. “Jeffrey may have to act more hardline to pacify the broader unionist base. People outside (the DUP) shouldn’t take an awful lot of comfort from a Jeffrey victory.”