On election eve, Joe Biden promised to be the “most pro-union president you’ve ever seen.” On Sunday night, he took a step toward that, issuing an unusual presidential video backing the right to unionize of workers at an Amazon plant in Alabama, and implicitly warning the online retail giant not to interfere. “Let me be clear: It’s not up to me to decide whether anyone should join a union,” Biden said. “But let me be even more clear: It’s not up to an employer to decide that, either.”