That issue has never been litigated. The expulsion power is in the same clause giving each House of Congress the power to make and enforce its procedural rules. This suggests that the expulsion power is limited to expelling a Senator or Representative who violates those rules. Reading this power to allow a House of Congress to expel a member for any reason would allow a House of Congress to perpetually deny a State of any representation by simply expelling its Senators/Representatives as soon as they are sworn in.
Powell v. McCormack narrowly read this clause regarding each House's power to judge a member's "Qualifications" for office. I suspect the Supreme Court would similarly read the expulsion power in a narrow way.
SC would likely not hear it at all. Like the impeachment of a president, it is acknowledged to be a strictly political question.