The UUP has branded "despicable" a letter circulated by the DUP claiming unless people voted for it, we could "go back to the bad old days" of the Troubles.
With just 48 hours left to polling in Thursday's Assembly election, the main unionist rivals continued to snipe at each other - in contrast to last year's Westminster race when a pact saw the UUP take two seats and the DUP regain East Belfast from Alliance's Naomi Long.
The latest row came as Mike Nesbitt hit out over "shocking" scaremongering by Arlene Foster's party.
He was referring to a letter circulated by the DUP, particularly in the North Belfast and Newry and Armagh constituencies.
"The DUP leader may have changed, but it's still the same old DUP selling the politics of fear and attempting to hold the electorate to ransom," Mr Nesbitt claimed.
"However, the latest letter from the DUP leader goes lower than any of the previous DUP communications by claiming that this country could 'go back to the bad old days' unless people vote for them.
"It is a shocking claim to make, even by DUP standards, who are so desperate to hang on to power they will stoop to making even more outlandish claims than before. The people of North Belfast and Newry and Armagh, where some of these letters have been distributed, suffered horribly at the hands of terrorists, and for the DUP to claim that we could 'go back to the bad old days' unless people vote for them is absolutely despicable." Mr Nesbitt added he did not believe the electorate would be "fooled", and would see through the letters.
However, East Antrim DUP candidate Alastair Ross hit back.
"We thank the UUP Press office for publicising our election material," he said.
"The 'bad old days' in the letter were when the UUP was in the lead but Sinn Fein set the agenda, prisoners were released wholesale and the RUC was abolished.
"Mike has said he wants to take us back to those days of (former UUP leader) David, now Lord, Trimble in 1998.
"Clearly Mike Nesbitt is feeling the pressure because Arlene Foster has a positive five-point plan to deliver a stable and secure Northern Ireland.
"If the DUP is not the largest party on May 6 then the next Executive will have a nationalist majority and a Sinn Fein First Minister.
"That will take Northern Ireland in the wrong direction.
"Unionists are right to be fearful of Martin McGuinness's vision for Northern Ireland's future."