Suppose Sen. Leahy had punched VP Cheney in the Senate chamber. Would the Secret Service have arrested the Senator? What would be the charge?
Suppose the VP had punched Senator Leahy after a heated verbal exchange? Is the VP immune from arrest? Could the Sergeant at Arms remove the VP from the chamber?
In both cases, it would be classified as an assault, no? I think that if either one of them demanded that the other be arrested, it would happen. Although, I also see all the members in the Senate chamber, running in the other direction frantically whispering…’Ididn’tseenothin…Ididn’tseenothin!’ 
It could have been worse The caning of Charles Sumner
If Leahy had punched Cheney, I’m sure the Secret Service would have dealt with him very quickly.
Presumably the Secret Service would also have prevented Cheney from going after Leahy because it would also put Cheney in danger.
If it hadn’t been treason or breach of the peace, Leahy woulda walked.