If with the FairTax a large number of people tightened up on their spending habits it wouldn't be a crisis but it would begin to make the government re-think its spending plans when combined with taxpayer protests that are sure to follow since - keep in mind - many of these new taxpayers will never have (knowingly) paid even income/payroll taxes before; they won't like it, either.
But why should taxpayers revolt when they have more purchasing power?