Republican North Carolina Sen. Thom Tillis and Democratic Delaware Sen. Chris Coons are co-sponsoring a bill that would allow a judicial panel to reinstate Department of Justice-appointed special counsels such as Robert Mueller if they are fired without good cause, the senators announced in a statement today. The bill would specifiy that special counsels “may only be removed for misconduct, dereliction of duty, incapacity, conflict of interest, or other good cause, like a violation of departmental policies.” (Donald Trump has alleged publicly that Mueller is biased against him and, of course, fired FBI director James Comey while Comey was supervising the investigation into the Trump campaign’s ties to Russia.)

Tillis is not the only Senate Republican to have recently challenged Trump. Last week, Alaska Sen. Lisa Murkowski, Maine Sen. Susan Collins, and Arizona Sen. John McCain cast crucial “no” votes against the “skinny repeal” health care bill that Trump supported, while Judiciary Committee chairman and Iowa Sen. Chuck Grassley announced he would not hold hearings to confirm a new attorney general until next year if Trump follows through on his threats to fire Jeff Sessions.

Assuming that all 48 Senate Democrats support the Tillis/Coons bill, which is being called the Special Counsel Integrity Act, two more Republican votes besides Tillis’ would be required to pass it. (On that front, South Carolina Sen. Lindsay Graham has previously said he was considering introducing such a bill himself.) If it then passed the House—a big “if” given the lower chamber’s more pronounced right-wing lean—Trump would presumably veto it, which would mean two-thirds majorities in each chamber would have to vote for it to override the veto. The law would then probably face legal challenge from the president on the grounds that it unconstitutionally constrains his executive authority. And this is all assuming Trump decides he wants to fire Mueller in the first place.

Which is to say, we are a long way legally speaking from a court panel actually reinstating a special counsel. The fact that a Senate Republican is formally supporting Mueller over his party’s president, though, is still significant—and Trump will almost certainly interpret it as a personal betrayal requiring personal retaliation. Six a.m. presidential Twitter meltdown, here we come!