There are many here and elsewhere who think the Senate's power to expel him is a slam dunk if they can come up with the votes. It isn't.
It is clear that constitution gives the senate the power to expel a senator for misconduct while in office. But it is vastly, vastly less clear that the Senate has the power to expel a senator for circumstances that were known to the voters at the time of the election and where senators may disagree on the interpretation or weight that should be given to those factors. The Framers definitely gave powers to the Senate to boot scoundrels and perverts out, but they definitely did not intend those powers to be used such that a little club meets and decides whether the person a state has elected meets their approval. What could go wrong there? Well, you can be sure they knew and that is why power was vested in the people and their duly elected representatives.
I dont' really have an interest in a full blown discussion on this. I am just sayin. Anyone who thinks it is a slam dunk will find that it isn't or wont be if it gets far enough along to be discussed for real.
Again, I don't want to get into the weeds on it right now but as an aside I will say that a political party probably does have the authority to remove a person as their nominee according to their rules. But that does not automatically mean the person is removed from the ballot. Depends on state laws, where you are in the election cycle and so on.
As I said above, I think one of the big differences is whether the voters were aware of an issue at the time of the election. For example, if Weiner had not resigned, I think Congress would be within their duty and powers to bounce his sorry arse out. But if he gets out of prison and decides to run for Congress or Senate and the voters know about his past, then the voters decision is supreme I think. The voters may think he is redeemed and they forgive him, they may think he is perverted arsehole but still better than the other person, or it may not be an important issue to them. That's up to the voters.
Congress and the Senate do have the power to set qualifications but you have to lay those out in advance and they have to be constitutional and they have to apply to all. The club cant just meet and decide who is okay and who isn't.