In an interview with ITV News, David Cameron would not take up the challenge to repeat the words "I won't cut tax credits and child benefit".
The challenge was put to Mr Cameron after Labour said "everyone knows the Tories are planning to cut tax credits and child benefit".
Speaking to ITV News presenter Ian Axton, the Prime Minister wouldn't use those exact words. He said he wants to "keep" child benefit, not cut it, saying it's "absolutely vital" and a "really important part of families finances".
On tax credits he said he would freeze it for 2 years.
The Prime Minister has said he will be "praying in all sorts of different ways" on the eve of the closest election race in a generation.
On a visit to Bath the Conservative leader opened up to ITV News West Country presenter Ian Axton about his nerves during the last few days of campaigning.
Prime Minister David Cameron was greeted by hecklers as he arrived for a rally in Bath this afternoon for his final visit to the region before the General Election.
It's part of a strategy to target Liberal Democrat seats - but they had their own campaigner in the city - comedian and local resident John Cleese.
Bob Constantine reports:
First time voter Dan Atkins got more than he bargained for when he went to meet the Prime Minister in his home village of Norton-sub-Hamdon.
The 19 year old asked David Cameron for a selfie and was surprised when he said yes.
David Cameron says voters in the West Country could make the difference in whether the Conservatives or another party form a government after the general election.
He's been outlining his plans for the first 100 days after the election at a rally near Yeovil. He says it's one of 23 key seats he needs to keep his party in power. He's targeting other seats in the region too.
David Cameron has said the economy is the issue at the heart of the campaign as he continued on the election trial with a speech in Norton-sub-Hamdon in Somerset.
Mr Cameron set out the Conservatives' programme for government in its first 100 days if it is re-elected to power and said Labour should not be given a second chance with the country's finances.
The Prime Minister has chosen Corsham in Wiltshire to begin his election campaign as the West is an important battleground for all the parties at this election.
Four of the Conservatives' top 50 targets are here in our region, including Chippenham and Wells, lost to the Lib Dems last time, plus Somerton and Frome and Taunton Deane, where the Lib Dem MPs have stood down.
Labour also have four top targets - looking to recover seats lost to the Conservaitives last time in Stroud, Gloucester, Kingswood and South Swindon in particular. Shadow Chancellor Ed Balls will be in those areas today.
UKIP believe their best chance is in the Tory-held Forest of Dean, while on paper at least regaining Weston-super-Mare is a Lib Dem target, and the Greens have Bristol West in their sights having had a success at local elections there.
Thirty so-called 'fat cat' council officials in the South West earn more than David Cameron, according to a report by the Taxpayers' Alliance.
The Prime Minister is paid an annual salary of £142,500 for running the country. But thirty senior council staff in the region earn in excess of £150,000.
Cornwall Council has the most, with a dozen employees getting paid in excess of £100,000 in 2012-13.
And one director at Swindon Council gets paid a salary of £256,268. The Board Director is in charge of Transformation and Strategic Projects for the council.
Today the Prime Minister David Cameron visited the Oasis Academy Brightstowe in Shirehampton as part of his tour of the South West.
He spoke to students at the School's breakfast club. He was joined by Bristol Mayor George Ferguson.