Police are investigating threats to staff in the Ulster Unionist Party, according to reports.
UUP leader-in-waiting Steve Aiken confirmed on the BBC's The View on Thursday evening that threats had been made against party staff.
He said threats undermined "the entire principle of democracy", adding: "That's not the kind of thing we should be accepting."
The BBC reports that people have contacted the party's headquarters to complain about the UUP's refusal to form a pact with the DUP in North Belfast.
The broadcaster reports police have also been informed about threats made online.
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."
Unionists fear the a UUP candidate would split the vote in the constituency and lead to Sinn Fein candidate John Finucane winning the seat in the General Election on December 12.
Mr Aiken had previously ruled out an election pact with the DUP, but while appearing on The View last night, he refused to confirm if he will put forward a candidate against DUP North Belfast MP Nigel Dodds.
In a statement 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.
"For our part the DUP has a positive plan for Northern Ireland to deliver more for everyone building on the extra billions of investment we have already delivered in the past two years for health and education, roads and broadband.
"We will be putting forward that clear message over the coming weeks.”