Republicans calling for this say it will help the economy. It will, for some... those with larger incomes. Who it will not help is poor or middle class. If you have large medical bills, the government gets 30% on top before you even start to pay for the amount owed. 30% tax on food and medicine should really thrill senior citizens who have already lost prescription assistance.
According to a couple of articles I've read (below) the Republicans supporting this measure don't want to risk the issue being a hot topic during the campaign, so they'll wait until after the elections to continue pushing for this.
Bush: 'We ought to explore' dumping IRS for sales tax
A National Sales Tax No Vote
The Case Against Tax-Reform Leadership
Bush won't bite IRS in campaign
Google news national sales tax