The IGs have already been removed. He already broke the law. Requesting an explanation as to why he did it post facto changes nothing about that, ergo the GOP Senators are doing effectively nothing: the IGs are already gone.
The letter doesn't even allude to any consequences whatsoever, neither for the initial illegality nor refusing their request for explanation of the illegality.
What does bipartisan even mean? I've seen my state lose a republican congressperson who, while I disagreed with, called out trump on disrespecting branches of gov during first term and since been replaced with a pro-trump congressperson. The checks and balances are eroding and the citizen response has to be strong.
A. he already broke the law. The point in notifying in advance is for Congress to push back if they think the reason was inappropriate, as Inspector Generals are the watchdogs that report to Congress.
B. Do you really think he had "the substantive rationale, including detailed and case-specific reasons for any such removal or transfer" for 17 simultaneous firings days into his administration?