Barack Obama has been warned that his refusal to pay the traditional "street money" to local operatives to help get the vote out in Philadelphia today could cost him the crucial percentage points needed to knock Hillary Clinton out of the race for the White House.

In many of the city's poorer wards, the recipients look forward to these bonuses from Democratic officials - a hangover from the days of the party's old-fashioned machine politics - even though the amounts are relatively small, ranging from $50 to $400.

But as in other contests, Obama is relying on his own army of unpaid

volunteers to get the vote out. The Clinton team, meanwhile, is not saying whether it will pay out "street money".

There are 69 wards in Philadelphia and estimates suggest it would cost Obama $400,000-$500,000 to pay the 14,000 people normally required to help get the vote out.

Carol Ann Campbell, an integral part of the city machine, said she expected Obama to win the city, but his failure to pay could cost him the crucial margin needed to force Clinton out of the race for the presidential nomination.

In an interview with the Philadelphia Inquirer last week, Campbell defended the practice of "street money", saying: "We are a machine town." She added that there was nothing dirty about it. "The committee people and the ward leaders have to buy lunch for hundreds of people, otherwise they won't have good workers. They have to buy coffee, orange juice and doughnuts. That's just the way it is."

Since the start of the primary campaign last year, Obama has avoided using the Democratic machine, on the assumption that it had already been tied up by the Clintons, and instead built up his own volunteer network. He has encouraged his supporters to be self-sufficient, with volunteers bringing dishes into campaign headquarters rather than sending out for meals.

The different approaches have produced a clash of cultures in Philadelphia. Obama's team on the ground is being supplemented by thousands of young supporters who have travelled from Washington, New York and other neighbouring conurbations, watched warily by the locals, some of them resentful about being denied the "street money".

Jeremy Bird, Obama's Pennsylvania field director, told the Los Angeles Times that the campaign had faced a similar predicament in South Carolina over the traditional distribution of money: "We always said that we're not going to do politics the way it's always been done because it's always been done that way."