Back in the mid-1990s I worked for the President's Council of Economic Advisers, when Joe Stiglitz was Chair.
One of my tasks was to write up a briefing concerning the report issued by the Kemp Commission, which was led by former HUD Secretary Jack Kemp and at the behest of the Republican congressional leadership. (I can't find the report online, but here is a link to Bill Gale's assessment published in Tax Notes.) The Commission proposed replacing the existing federal income tax system with a single tax rate together with some personal exemptions. As usual with such proposals, the stated rate wouldn't have come close to current revenue levels, but never mind that--the Kemp Commission insisted that all sorts of wonderful things would happen if their proposal were adopted.
One day at a meeting with Stiglitz, after I'd had a chance to read the report, I explained that the Commission report claimed that the plan would double the GDP growth rate. Joe chortled, as he was wont to do, and declared: "Why didn't they just say triple!"
Rather than point out all the practical problems with Jeb! Bush's "proposal" to double GDP growth to 4%, I think Democrats, economists, and others in the reality-based community should take the same approach Joe did: just ask, why hasn't Jeb! promised 6%?
Actually, why not 8%?