I think UBI and flat tax are fundamentally incompatible. Reasonable levels of UBI require high taxes, and doing so with entirely flat taxes will be quite punitive on the middle class. I think the vast majority of the population think there is a lot of merit to graduated tax rates as well, charging 50% tax on all of someone's income above $10,000,000 isn't as punitive to any sort of living standard as charging 50% tax on someone's first $30k of income.

If we want a UBI of $24,000/year ($2,000/month) for an adult population of 250 million we are talking 6 trillion dollars in additional government spending. Assume for a minute we can cut 2 trillion in spending so we are talking about a spending increase of 4 trillion. This would mean roughly doubling current government revenues. If you wanted to do so with a flat tax, you'd see rates near the top rate now on all income.

Are people ready for a world where you get $24,000/year from the government and pay something like a 40% tax on all your income? Compared to the current tax code you probably break even at roughly $75,000/year and for incomes above that you'd pay more in tax.