Leaders in London and Belfast could face leadership challenges after Thursday's vote.
A by-election in Northern Ireland appears to have opened a gaping hole in the foundation of the April 1998 Good Friday peace agreement.
Analysts say First Minister David Trimble is facing a fight for his political life after his Ulster Unionist Party, Northern Ireland's largest political group, suffered a surprise defeat on Thursday for a seat in Britain's House of Commons. The UUP candidate lost out to the Democratic Unionist Party, a more radical pro-British group that opposes the accord.
The loss also threatens plans to reform Northern Ireland's heavily Protestant police force. The Royal Ulster Constabulary (RUC) is to be renamed, and efforts are under way to recruit more Catholics into its ranks.
Trimble, co-winner of a Nobel prize for his work on the peace accord, promised over the weekend to "fight on." Britain's Northern Ireland Secretary Peter Mandelson said the government was "determined to carry through reforms of the RUC."
But the Rev. Ian Paisley, the DUP leader, claimed his party had "taken over the mantle of Unionism," and said a "discredited" Trimble "must go."
"Trimble faces concerted opposition to his leadership of the UUP from Unionist hardliners," says political commentator Christopher Walker. "Ian Paisley's DUP will now target other seats held by the UUP, including Trimble's own." The UUP holds eight seats in Britain's Parliament, and a majority in Northern Ireland's self-rule assembly. The UUP's Jeffrey Donaldson, who opposes the peace process and challenged Trimble's leadership in the past, said his party is in "electoral meltdown."