A national sales tax large enough to replace income taxes will encourage more investments in building manufacturing facilities abroad and purchasing commodities from other countries. Only those unable to make their purchases on the internet, from a Chinese mail-order catalog, or a burgeoning black market will pay the tax. This tax would only work using imposition of tariffs on all incoming trade.
All of the already-taxed money that retired people have put away would suddenly be subject to the new sales tax, and instead of auditing your individual tax return, now the feds are poking into every local retail establishment. Even more businesses will want to be paid in untraceable cash. All efforts by other countries that have tried to impose a sales tax above 10% have been met with massive evasion. If you think the economy is in the toilet now, a national sales tax would drop it into the sewer.