On a tangent to the initial question posed in How can a Senator be removed from office during a term for medical reasons? the possiblity that a sitting member of the Senate might be "impeached" has been suggested. The example raised was that of Senator William Blout. In the early days of the Senate (being only 5 years old at the time) When Blout's behavior was deemed 'actionable' the House voted to Convene Impeachment Hearings (7/8/1797 41-30 for). The next day the Senate "effectively" expelled him [sequestered], making any such House hearings irrelevant. The raises the question: "Can members of Congress be impeached?"
In the comments to the previous questions (growing ever more off-topic, thus this new question) @RBarryYoung posits that
"Congress determined some time ago that members of congress are not subject to impeachment"
How is this determination "Manifest"? Is it merely a "Rule of House/Senate" (each house may govern there own affairs, but still such rules are not necessarily "law". As we have seen those rules can be easily ignored at the convenience of the majority) Or did a bill get passed (and signed) exempting Congress from Impeachment? Or is it just 'not done' by tradition?