The few television viewers willing to subject themselves to CNN’s “Climate Crisis Town Hall” now understand Democrats’ ambition to dismantle U.S. energy production. Many consumers also know their health plans would be outlawed under a bill authored by Sen. Bernie Sanders of Vermont and endorsed by Sen. Elizabeth Warren of Massachusetts. But voters and investors may still struggle to tabulate the full cost of a potential new activism in Washington—including a reversal of President Donald Trump’s drive to cut federal red tape.

Andy...