Donaldson’s time in politics began in 1985 when he was elected for the UUP to represent the South Down constituency in the 1980s iteration of the Northern Ireland Assembly.
Before his election, he had worked for South Down MP Enoch Powell who had joined the Ulster Unionists from the Conservatives in the 1970s.
In 1997, when his boss James Molyneaux retired as the Lagan Valley MP, Donaldson retained the seat comfortably for the UUP.
But his unhappiness with the party became obvious as political negotiations at Stormont moved towards the signing of the 1998 Good Friday Agreement.
He became one of the UUP’s biggest critics of the deal over issues like the IRA’s decommissioning of weapons, warning his then leader David Trimble against supporting it.
Even after the agreement, and the first power-sharing government was formed in its wake, he continued to pose problems for the party and supported failed bids by others to oust Trimble from office.
In 2003, he quit and defected to the DUP.