Plans which would cap the benefits a family can receive at the level of average income face a tight vote in the Lords later.
While the cap, which would limit payments to £26,000 a year per household, is likely to pass with Labour support, it is opposed by many Bishops and former Liberal Democrat leader Paddy Ashdown.
Iain Duncan Smith has backed his department's plans, saying they would not worsen poverty levels in the UK. Lib Dem leader Nick Clegg has also supported them.
The policy polls well with the public but campaigners warn it will force those who lose their jobs to endure grinding poverty.