Avery No it doesn't. Google "how many us senators are independents" You will find there are 49 republicans, 48 democrats and 3 independents. For some reason I can't easily find how many, if any, independent us representatives there are.
Yes, "automatically" was a bit of an exaggeration. It would certainly be possible that an independent congressperson would not be so firmly on one side of the two-party divide that he can essentially be considered a party member in all but title. I could not imagine Senator Sanders voting alongside the Republicans unless there is a major shift in the party landscape. Similarly, it would not be impossible for a third party candidate to make it to the House either, it just does not happen very often.
Contrast Germany, where independent candidates are not really much of a thing, if someone wants to run without an established party backing them the way to go it to form a small party. Since the adoption of our constitution back in 1949, there has been a grand total of one Bundestag (elections happen on roughly a four-year cycle, barring re-elections which have happened a grand total of twice so far) where a single group has had a full majority, and even that was technically not a single party, but a close alliance of two parties that are regionally separated and rarely compete with each other - in a way the closest equivalent to the independent senators caucusing with the democrats that Germany has.
Also, for the office of president of the Bundestag, those two parties are counted as one for "largest party" concerns - ultimately it is taken for granted that those two will be working together, whereas the coalitions that are used to form a majority are usually subject to open-ended negotiations after every election.
