is the number of groups he would piss off.
On Morning Joe this morning three areas were mentioned as necessary for him to come close by closing loopholes in order to achieve the revenue he would need just to offset the loss of revenue from cutting tax rates.
1. Home mortgage interest deduction - you will piss off a reliable Republican group, realtors, because demand for house will go down significantly, which will depress the sales prices of the houses being sold. This will unfortunately put even greater economic pressure on local governments, whose primary source of revenue is taxes on the value of real property. In most jurisdictions the assessed value on which those taxes are calculated is heavily influenced by sales price. Absent demand you will see a fall-off in housing starts, which would economically be disastrous, also affecting durable goods - absent housing starts, fewer refrigerators and washers and driers and hot water heaters would be needed, diminishing demand in manufacturing.
2. Deductibility of State and Local taxes of any kind, whether sales or income. For one thing, this will violate some conservative economic purists, because you will then be subjecting income to double taxation. For another, this will fall most heavily on upper middle class people, especially in states with higher tax rates. The impact of this might not only flip a few states electorally, it has the potential of costing Republicans House seats in blue states like New York which have higher state and local tax rates.
3. The Charitable contribution deduction. The first to scream will be non-profits, who depend upon the leverage of that deduction for much of their income. Remember that the boards of many museums and the like tend to be wealthy people. They will worry about the viability of their institutions. The impact would also be felt by middle class and lower people as the cost of tuition at colleges and universities might have to escalate to offset the loss of revenue from contributions towards endowments, that further hurting with the loss of additional funds for scholarships.
In short, if analysts like Steve Rattner and Ezra Klein are right, the specifics of Romney's tax plan could potentially be disastrous not only economically - because the numbers do not make sense - but also electorally. If these three items are part of Romney's plan, they have the potential to destroy the Republican party.