Facebook Inc.’s Libra cryptocurrency gained a pair of powerful skeptics ahead of a congressional hearing next week on the social-media giant’s ambitious plan to create a world-wide payments network.

President Donald Trump late Thursday said Libra “will have little standing or dependability,” in a series of tweets slamming bitcoin and other cryptocurrencies. Earlier in the day, Federal Reserve Chairman Jerome Powell told the Senate Banking Committee that Libra “raises a lot of serious concerns, and those would include around...