Sinn Fein and the DUP have been told they need to stop "hypocritical mudslinging" if upcoming talks chaired by US diplomat Richard Haass are to be successful.
Speaking on Saturday at a meeting of party members, the South Belfast MP urged fellow political leaders to be "measured in their words as we prepare for the start of the Haass negotiations and not say anything that will sour the political process any further".
He served as US envoy to Northern Ireland from 2001 to 2003 and was the agreed choice among the five parties represented in the Executive. The all-party group will try to bring forward a set of recommendations by the end of this year on parades and protests, flags, symbols, emblems and other related matters.
Dr McDonnell described the arrival of Dr Haass as the "last chance saloon".
"Political leaders and others must, at this critical time be, measured in their words as we prepare for the start of the Haass negotiations and not say anything that will sour the political process any further," he said.
Attacking the DUP and Sinn Fein, he said: "People are fed up with the hypocritical mudslinging by the DUP and Sinn Fein we witnessed over the summer months.
"No one political party or community has a monopoly on being offended or being re-traumatised. To make progress we require leadership and firm proposals, not a rift between the First and Deputy First Minister exacerbated by letters from America and commemoration speeches. The SDLP, will not be found wanting in resolve, realism and responsibility."
At the meeting, Dr McDonnell confirmed that the SDLP's two negotiators will be MLAs Alex Attwood and Conall McDevitt.