Looked it up and it has happened once - a Senator Blount in 1779 but it's kind of murky...
The 1799 impeachment of Tennessee senator William Blount stalled on the grounds that the Senate lacked jurisdiction over Blount. Because, in a separate action unrelated to the impeachment procedure, the Senate had already expelled Blount, the lack of jurisdiction may have been either because Blount was no longer a senator, or because senators are not "civil officers" of the United States who are subject to impeachment. At any rate, no other member of Congress has ever been impeached. Of the remaining cases, two did not come to trial because the individuals had left office. Each of the seven Senate convictions has involved a federal judge: in the most recent such case, Alcee Hastings subsequently gained election as a member of the House of Representatives, which had impeached him.
The recall is certainly the better route but I don't know if it's possible with federal elected officials.
I wonder if this is still the case - i which case the answer is no, senators can not be impeached.
Good catch.
If an attempt was made to impeach Senators, they would be trying their own cases.
A better solution would be to demand that all involved be charged for violating the Smith Act of 1940, and removed from office after trial and conviction of the charges.