Police are investigating threats against Ulster Unionist Party (UUP) staff.
It followed the refusal of its incoming leader to agree a General Election pact with the Democratic Unionists.
The DUP has also condemned the alleged intimidation.
Deputy leader Nigel Dodds’ North Belfast seat is targeted by Sinn Fein, and the party wants to secure a deal which would see him run unopposed by a UUP candidate.
A UUP statement said: “Our headquarters had received a number of calls of a threatening nature and we have alerted the Police Service of Northern Ireland to that fact.”
Incoming UUP leader Steve Aiken has ruled out a pact with the DUP in a break with previous unionist practice designed to maximise the number of pro-union MPs returned.
Mr Dodds said: “There have been some reports of intimidation and threats issued towards members and activists from the UUP.
“Any such incidents are deplorable and must be totally condemned.
“The only appropriate way to express your views and the most effective way to send a message in our society is through the ballot box.”