until 1969.
https://www.hklaw.com/PoliticalLawBlog/When-Can-Congress-Refuse-to-Seat-a-Duly-Elected-Candidate-12-12-2017/?utm_source=Mondaq&utm_medium=syndication&utm_campaign=View-Original
It is not about what congress has the legal authority to do but what it has the power to do.
I would argue that many of the things done by all three branches of the federal government today are outside their legal authority but no longer outside their power.