Absolute criminal immunity for presidents is a non-starter for everyone except Donald Trump. No sane person believes it’s either constitutional or even a good idea.
Even so, there is a lot of discussion about some sort of immunity for presidents based on an analogy with the civil immunity that presidents do enjoy. But that’s based on sloppy reasoning. In fact, courts don’t have the power to confer any type of presidential criminal immunity at all.
Criminal immunity is a fundamentally different issue than civil immunity because, at least with respect to violations of federal law, only one person can bring an action, the attorney-general. And the attorney-general derives his or her authority from the presidency itself. In other words, whether or not an ex-president should be prosecuted is ultimately up to the current president.
Since bringing criminal charges is firmly within the executive branch’s constitutional remit, whether the judicial branch can interfere with those decisions before they are made has serious separation-of-powers implications. In fact, whether to grant an ex-president criminal immunity checks four of the six boxes in Baker v. Carr (69 U.S. 186 (1962)) – and checking any one of those boxes is enough to make an issue a political question beyond the reach of the courts.
None of this is theoretical. We've seen it happen. After Richard Nixon resigned, President Ford, after careful consideration, used his constitutional authority to issue Nixon a pardon for his role in the Watergate scandal. "As President, my primary concern must always be the greatest good of all the people of the United States whose servant I am. . . [O]nly I, as President, have the constitutional power to firmly shut and seal this book. My conscience tells me it is my duty, not merely to proclaim domestic tranquillity but to use every means that I have to insure it."
Ford could certainly have decided that "the greatest good of all the people of the United States" required Nixon to be prosecuted for his crimes. At the time, many people thought he should be. Had Nixon used Seal Team Six to assassinate someone, Ford might have reached a different conclusion. And that would have been his call to make. Employing prosecutorial discretion and the pardon power to strike that balance is a matter committed to the executive branch, not the courts.
In short, whether a president should be criminally charged for acts committed in office is a non-justiciable political question. At least when it comes to federal charges, courts are unable to grant the president any immunity at all, qualified or otherwise.