You want to keep you investment of time, money and emotional capital to an absolute minimum, just in case what seems like a wonderful idea turns out not to be.
Even with compensating income transfers to poorer people, who benefit in absolute terms less than the better-off from the tax relief, the idea is politically unpalatable.
However, Tory MP Sir Roger Gale says the idea is simply incompatible with the role of an MP and is "parliamentary populism and opportunism at its absolute worst".