As U.S. Democrats lay the foundation of their bid to unseat President Donald Trump in 2020, raising taxes on the rich is emerging as a central theme for a party being pulled to the left by its invigorated self-described progressive wing. Ideas include a “wealth tax” on assets, much higher income tax rates on the biggest earners and a new tax on financial trades, all in the name of raising more money for new government programs while making a dent...