This article is part of the Opinion Today newsletter. You can sign up here to receive the newsletter each weekday.

The key to a legitimate Russia investigation has always been Republicans not named Donald Trump.

In Congress, Republicans — and only Republicans — have the power to make clear that Trump’s presidency is effectively over if he fires Robert Mueller, the special counsel. At the Justice Department, Republicans can refuse to carry out any order to fire Mueller and create a firestorm if Trump subsequently fires them. And at the White House, Republicans serving under Trump can explain to him that he would create a true constitutional crisis if he tried to subvert the rule of law.

We now know — thanks to Michael Schmidt and Maggie Haberman of The Times — that Trump did try to subvert the rule of law. In June, he issued an order to fire Mueller, with three silly justifications (including the fact Mueller once quit Trump National Golf Club in Sterling, Va.).

But the White House counsel, Donald McGahn (pronounced “McGann”), refused to send the order to the Justice Department, which would have had to carry out any firing. Removing Mueller, McGahn told other White House officials, “would have a catastrophic effect” on Trump’s presidency. As McGahn expected, Trump then backed down.