In 2007 circumstances changed, and Mr. Donaldson also altered course. Following talks in Scotland, the D.U.P.’s veteran founder and leader, the Rev. Ian Paisley, ended his opposition to the Good Friday Agreement, and the D.U.P. went into coalition, in Belfast’s devolved assembly, with Sinn Fein, formerly the political wing of the I.R.A. Mr. Donaldson, while keeping his seat in the London parliament, was also a member of the power-sharing Belfast assembly, where he served for a year as a junior minister.

His time as leader of the D.U.P. was dominated by his response to Brexit.

In 2021 he became D.U.P. leader after infighting brought down its two previous leaders, Arlene Foster and Edwin Poots, within weeks of each other.

Although Northern Ireland had voted, overall, to remain in the European Union in the 2016 Brexit referendum, Mr. Donaldson, leading the D.U.P. from Westminster, aligned unionism with the most hard-line Brexit faction in the British Conservative Party. Led by then Prime Minister Boris Johnson, this group wanted a complete withdrawal from all European Union institutions, including customs, manufacturing, environmental and veterinary protocols that allowed free trade and movement across the Irish border.

In Ireland, and abroad, a frictionless Irish border was widely seen as essential to preserving the peaceful status quo that followed the Good Friday Agreement. So when the U.K. government agreed, under heavy pressure from the E.U. and the United States, to impose customs and trade checks between Northern Ireland and the rest of the United Kingdom, rather than on the Irish border, Mr. Donaldson protested by using the D.U.P.’s power-sharing veto to collapse the devolved government in Northern Ireland in 2022.

After two years of Northern Ireland having no functioning government, and with his party losing support to hard-liners and moderates on either side, Mr. Donaldson finally agreed to revive the assembly in January this year.