Mayor Michael R Bloomberg pulled out a narrow re-election victory on Tuesday, as voters angry over his manoeuvre to undo the city’s term limits law and his extravagant campaign spending provided an unexpected lift to his challenger, William C Thompson Jr.

Unofficial returns showed Bloomberg with 51 per cent and Thompson with 46 per cent. The result will make Bloomberg only the fourth three-term mayor in the last century.

“Conventional wisdom says historically third terms haven’t been too successful,” the Mayor said at the Sheraton New York Hotel in Midtown Manhattan after a tense night of watching returns. “But we’ve spent the last eight years defying conventional wisdom.”

The margin seemed to startle Bloomberg’s aides and the city’s political establishment, which had predicted a blowout. Published polls in the days leading up to the election suggested that the Mayor would win by as many as 18 per centage points; four years ago, he cruised to re-election with a 20 per cent margin.

Bloomberg had based his third-term campaign largely on the argument that the city has been better run since he ushered in an era of corporate efficiency and nonpartisan leadership at City Hall. He also pointed to his accomplishments in education, crime reduction and public health.