Mr Campbell accused Reform UK of being the "paymaster for unionist division"
The UUP leader said the plan backed by the DUP and TUV would not be able to represent differing positions among the parties.
On Sunday TUV leader Jim Allister said he would back a non-party candidate who could embrace the support of all unionists
Sinn Fein’s Michelle Gildernew won the seat by just 57 votes at the last general election in 2019.
Gavin Robinson says Jonathan Buckley “next generation advocate” for unionist voters in the area
The seat is currently held by Sir Jeffrey Donaldson, the party’s former leader, who is charged with a number of historical sex offences.
Henry Reilly tells outgoing Chair "go raibh maith agat Ceann Comhairle".
Gavin Robinson also vowed to pursue a ‘positive’ campaign over the coming six weeks.
Northern Ireland parties are beginning preparations for the July 4 poll.
DUP peer, a former speaker of the NI Assembly, was born in Donegal and is not automatically entitled to British citizenship