The problem with this agenda is that there is little evidence that Mr. Trump did commit crimes as president. A conviction, given what we know now, is all but impossible. The calls to investigate him echo the president’s own calls to investigate Barack Obama, Hillary Clinton and Joe Biden based on mere speculation — calls that most people, especially liberals, rightly condemned.

The most plausible charge is that Mr. Trump obstructed justice by interfering with, and possibly lying to, Robert Mueller and his investigators. Critics also argue that Mr. Trump may have broken the law by threatening to withhold military aid to Ukraine unless the Ukrainian government announced the opening of an investigation into the Bidens. A third possible charge is that Mr. Trump corruptly mixed his financial affairs with government business.

All of these charges would face formidable difficulties in court. No former president has ever been prosecuted for crimes committed during his tenure. Courts tread cautiously when new legal ground is broken, worried about upsetting reasonable expectations about what the law is. And judges interpret criminal laws strictly because the defendant’s freedom is at stake.