President Donald Trump may soon have a chance to prove wrong the notion that economics will kill the U.S. coal industry and keep clean energy thriving. Two initiatives pending in Washington—one to prop up large traditional power plants and a second to impose tariffs on solar panels—could let Trump upend wholesale electricity markets and tip the advantage away from renewables.

The moves, which both invoke laws that haven’t been used in a decade, come as Congress begins debating a White...