Wealth-tax proposals (e.g. Warren's 2% on net worth over $50M, Sanders' 1% over $32M) target the top 0.1% of households. Some European countries have had wealth taxes; most have repealed them, citing administrative complexity, capital flight, and undervaluation of illiquid assets.
Constitutional questions: the 16th Amendment authorizes income taxes, and the Supreme Court has not ruled on whether a national wealth tax counts as a "direct tax" requiring apportionment. Most analysts believe a wealth tax is constitutional but litigation is likely.
Practical questions include: valuation of illiquid assets (private companies, art, real estate), enforcement against offshore wealth, liquidity for those whose wealth is in non-cash assets, and revenue estimates that vary widely with avoidance assumptions.