If President Trump tries to stop the Russia probe by firing Robert Muller or by issuing pardons, the House must impeach him and the Senate must remove him from office.
This is not complicated. It does not matter what happened in the past. A President who uses his powers this way cannot be trusted in the future.
The message that shutting down such an investigation sends to people around the president – his family, his business associate, his staffers – is that they can act with absolute impunity because he will protect them absolutely. Even if there was no collusion with a foreign power, no bribery, no shakedowns, no hacking, no corruption, and no obstruction of justice, there will be. A pardon pen that has been used once can be used again. Everyone knows that if the investigations start up again they will be shut down again, and some of them will act accordingly.
An executive branch whose members know they operate completely outside the law is not a presidency; it is a dictatorship. A presidential guarantee of immunity for political and personal crimes is completely incompatible with American democracy.
Republicans cannot indulge in waffling about whether their policy agenda is worth it. No policy agenda is, because sustained executive lawlessness destroys the structure that makes any policy agenda possible. And Democrats cannot indulge in waffling about whether a President Pence (or President Ryan, etc.) would be better or worse. Any President who respects the rule of law and lets investigations run their course would be better.
This is fundamental to the American political system, and it does not depend on whether you think Muller’s investigation has found anything criminal or is likely to. It is about having a President who can do the most basic thing the Constitution requires of him: “take care that the laws be faithfully executed.”