At the moment there are probably less votes dependent on the defence budget than there are for the NHS or education budgets. The Tories are also protecting the foreign aid budget in order to show how different the are from the 1980 and 1990's vintage nasty Tory party. However as a result of protecting the NHS, education and foreign aid deeper cuts will be required elsewhere else including for example defence and the police. The trouble is every other party is pledging almost exactly the same policies.