Colum Eastwood has become the first serving leader of the SDLP party ever to address the Ulster Unionist Party conference.
Eastwood used the speech to respond to critics who have questioned whether the two parties can cooperate to form a viable alternative to the Democratic Unionist/Sinn Féin administration.
"Of course we can," Mr Eastwood told the event in Belfast.
He insisted obvious differences should not stand in the way of the parties uniting on issues where they shared common ground.
In a speech that drew a standing ovation from conference delegates, the Foyle MLA claimed the "cosy establishment" running the Executive had failed the people and eroded trust in the effectiveness of the devolved institutions.
"The biggest challenge of the official opposition is to begin the process of proving that our devolved institutions contain a power and importance beyond the maintenance of peace," he said.
"True reconciliation here will not be achieved through warm words but through practical politics, a politics which fundamentally changes the economic and social patterns of this society."
Highlighting the UUP and SDLP's key role in forging the 1998 Good Friday peace agreement, Mr Eastwood claimed the DUP and Sinn Fein were, by contrast, devoid of "ambition and aspiration".
"They believe the symbolism of their coalition suffices, and offer nothing more," he said.
"They're all guff and no government."
He added: "Together we must break up and break down that cosy establishment.
"We do that by building trust and credibility across this society. We do it by embracing the politics of partnership and cooperation. Let's be honest - we are not there yet.
"We have work to do and that work goes on. Our success can permanently transform the politics of this place - old battles of identity will be replaced by a new battle of ideas.
"For me, for the SDLP, that's work that's well worthwhile."
While Mr Eastwood highlighted the common ground shared by the parties, he also touched on their differences.
Both parties campaigned for Remain in the EU referendum, but they have adopted different stances in the wake of the result, with the UUP essentially accepting that Brexit is happening while the SDLP insist Northern Ireland should be able to maintain its link with the European Union.
"Looking ahead the future offers many choices - but the only future worth knowing is the one which chooses co-operation," Mr Eastwood told the conference.
"It is that very sentiment and its substance which has always been at the heart of the European Union.
"Building common ground and common cause is the only effective weapon in the face of the divisiveness which threatens the consensus of decency that is the centre ground. That is why the SDLP is so determined to retain the benefits of the EU and its membership across this island."
On the constitutional question, Mr Eastwood acknowledged both parties would not see eye to eye, but he challenged the respective memberships to engage with each other on their competing visions of Northern Ireland's future.
"My appeal is this - try to convince us of your vision for the future and we'll try to convince you of ours," he said.
"Let it be a discussion based on hard facts and hard truths. Most of all let it be creative - and then in time let the people decide. That's the way politics is supposed to work.
"It's how it works at its best - without threat or theatrics."