Welfare (pensions, OOW benefits etc) and the NHS are paid for solely by NI contributions. The government is not allowed to touch this money but it may borrow from any surplus; the yearly surplus is around £2bn.
The coalition rhetoric of "taxpayers are sick of seeing blinds closed when they go out to work" and all the other rubbish they spout is either ignorant stupidity or malevolent divisiveness, because tax has nothing to do with it.
NI contributions may not be used for any other purpose than welfare, so why does the government want to cut benefits and introduce private pensions for everybody? Gideon Osborne spoke in 2011 of possibly combining tax and NI. This would give him access to a huge pool of money that is specifically earmarked, by Statute, for healthcare and welfare - he would want to use it for other purposes.
Tax credits, on the other hand, do come out of the tax budget; however this money doesn't subsidise poorly paid workers, it arguably subsidises businesses so they do not have to pay a living wage.
Basically, I don't see how the government can legally include the welfare budget in its programme of cuts. There may be an argument for making the welfare system more efficient but any cuts or changes would not affect the government's budget.