The Alliance Party has said its staff have been subjected to "many abusive and nasty" calls after the distribution of a leaflet about Union flag policy.
The leaflet claims Alliance has sided with Sinn Fein and the SDLP to stop the Union flag flying at Belfast City Hall on all but a few days each year.
Up to 40,000 of the leaflets have been distributed in a joint operation carried out by DUP and UUP activists.
Alliance councillor Maire Hendron said she was "disgusted" by the leaflet.
She accused the DUP in particular of "underhand tactics" ahead of a council vote on the issue.
However, in response to complaints from Alliance, the DUP said it has "nothing to apologise for".
The UUP has also confirmed its councillors were involved in distributing the leaflets and said flag policy was an emotive issue for unionists.
The leaflets are printed in the Alliance party's distinctive yellow colours and include the telephone numbers for its headquarters and east Belfast office.
Over the past two weeks, they have been posted through doors across Belfast, ahead of an upcoming debate on flag policy that is due to take place at the council next month.
Alliance Councillor Maire Hendron said: "I am absolutely disgusted at the level some of my counterparts on Belfast City Council have stooped to on this matter."
She also demanded an apology for the claims contained in the leaflet.
In a statement, she said: "I cannot be clearer when I say the Alliance Party has not formed a pact with Sinn Fein or the SDLP over the future of the flag at Belfast City Hall.
Ms Hendron added: "The Alliance Party has always had a policy of designated days when it comes to flying the Union flag over the City Hall.
"Not only is this the status quo at Stormont - where the DUP's MLAs make no complaint - but it is in line with the recommendation of the Equality Commission.
"We believe to fly the flag as an important symbol on certain days makes it a stronger emblem for our city."
DUP councillor Christopher Stalford said the flying of the Union flag outside Belfast City Hall was an issue a lot of people took seriously.
Referring to the upcoming vote, he said the political and mathematical reality was that Alliance could make a difference to whether the flag continues to fly.
Mr Stalford added that the leaflets asked anyone contacting the Alliance to be "respectful at all times".
He expressed astonishment that the party should complain about being contacted by voters who want to discuss its policy on an important issue.
A UUP spokesman said anyone who was at last month's event to mark the centenary of the Ulster Covenant in the Stormont estate would have noted the dismay shown by those attending at the lack of a Union flag on display at Parliament Buildings.
He added that the UUP wished to reiterate the request that anyone contacting politicians on the issue should be respectful at all times.