If you take the GOP at it’s word, you can not investigate Trump for anything he did before he became president-that’s a witch hunt. You can’t investigate Trump for anything he did while in office because that is subverting the will of the voters. You also can’t take action once he leaves office, it’s too late to matter.
Sure, they investigated Bill Clinton for White Water, which happened before he was in office. They investigated Hillary for things that happened while she was in office. And they investigated Biden and his son for stuff that happen when Biden wasn’t in office anymore. It’s clear they love standards so much they insist on having at least two for any occasion.
But that’s not the point. Republicans have made it clear that you can investigate Democrats for anything. Even shit they made up. And you can attempt to take legal action against Democrats for anything, even shit they made up.
So, according to Republicans-when can you investigate a fellow Republican? When can you take action against a fellow Republican?
At its winter meeting, the Republican National Committee overwhelmingly voted to censure Cheney and Kinzinger, the sole Republican members of the select committee investigating the attack on the Capitol. Kinzinger, like several other Republicans who voted to impeach Donald Trump, has already announced that he won’t seek another term. But after today’s resolution, the Republican Party will officially back Cheney’s primary challenger.
Source: Republicans Formally Censure Cheney and Kinzinger for Investigating January 6 – Mother Jones
So you can take action against a fellow Republican in one and only one situation. I call it “Catch 1/6”. If they dare to cross the line and hold another Republican responsible for anything that happens on 1/6, you can kick them out of the party.