what challenge do policy-makers face when contemplating large changes in spending levels for the federal budget

Negative view/pressure from the public

Explanation:

In general, it took a considerable amount of time before The benefits from Large changes in spending levels for the federal budget can be felt. (it acted as some sort of future investment).

Since the new program most likely require an increase in federal budget, the government tend to increase the tax rate from the citizens to cover for the expense.

Since the public cannot felt the benefit at the current time, The Government will have to face negative pressure from the public and have to work extra hard to convince the public that they will benefit from the new changes in the future.