London ? Northern Ireland’s largest pro-Catholic political party voted overwhelmingly Sunday to cooperate with the predominantly Protestant police force, a remarkable reversal that was widely seen as a critical step toward cementing peace in a British province recovering from three decades of sectarian war.

“Today you have created the potential to change the political landscape on this island forever,” Sinn Fein leader Gerry Adams told about 2,000 party members after the nearly unanimous show-of-hands vote at a Dublin conference center.

The vote by Sinn Fein, the political affiliate of the Irish Republican Army, which waged a bloody struggle to free Northern Ireland from British rule, was a required step toward restoring a Catholic-Protestant power-sharing government in the province. The British government has given the bickering parties in Northern Ireland until March 26 to form a local government or see the province’s affairs fully controlled by the central government in London.

Ian Paisley, the tough-talking leader of the largest Protestant party, the Democratic Unionist Party, and Sinn Fein leaders have sparred over several issues related to governing together, but none has been more full of anger, passion and history than policing. Catholics have long argued that the police have been a corrupt partner with Protestant paramilitary groups in systematically discriminating against them, often to the point of disregarding beatings and murder.

Sinn Fein president Gerry Adams votes to support Northern Ireland's policing during a special conference on supporting the Police Service of Northern Ireland in Dublin, Ireland, on Sunday.

Catholic mistrust of the police was underscored last week by a report, issued by the independent police ombudsman, that concluded that police had colluded with Protestant paramilitary informers and protected them from prosecution even when they were implicated in murders and other violent crimes.

The report cast new doubt on whether Sinn Fein members could put aside their deeply held animosity toward the police and vote to endorse them – a key goal of the landmark 1998 Good Friday peace agreements that called for Catholics and Protestants to share power to end a war that cost more than 3,600 lives.

Paisley made no immediate public comments concerning the Sinn Fein vote. In the past, he has said he would cooperate with Sinn Fein only if the party endorsed the police and demonstrated support for law and order. Paisley has been deeply skeptical of previous peace overtures, including the IRA’s 2005 declaration that it had laid down its weapons permanently and renounced violence in favor of political solutions to the province’s problems.

A spokesman for Tony Blair said the British prime minister “welcomes this historic decision and recognizes the leadership it has taken to get to this point.”