A number of people have argued that President Trump could not be convicted of the crime of obstruction of justice for firing James Comey, whatever the facts ultimately show about Trump’s motive. David Rivkin and Lee Casey, in today’s Wall Street Journal, are the latest to make this argument. In their words:

Mr. Trump’s critics claim that subjecting the president’s actions to scrutiny as potential obstructions of justice is simply a matter of asking judges to do what they do every day in other contexts—determine the purpose or intent behind an action. This is also wrong. The president is not only an individual, but head of the executive branch. Separating his motives between public interests and personal ones—partisan, financial or otherwise—would require courts to delve into matters that are inherently political. Under Supreme Court precedent stretching back to Marbury v. Madison (1803), the judiciary has no power to do so. And lawmakers enjoy an analogous immunity under the Speech and Debate Clause.

Yet Rivkin and Casey contradict this claim—that judges are not permitted to inquire into the motives of the president—a few paragraphs later. There, they agree that the president can commit the crime of bribery. Like many people, they don’t think the president can be tried for bribery until after he leaves office, but that is not the issue here.

The problem for the argument is that a president can commit the crime of bribery only if he has the right motive—or what is sometimes called “corrupt intent.” Imagine, for example, that Michael Flynn had offered Donald Trump a generous campaign contribution and at the same time asked Trump to call off the FBI investigation of him, which Trump subsequently did. To determine whether Trump illegally accepted a bribe, a jury would need to be convinced that Trump called off the FBI investigation in return for the money—in other words, that Trump was motivated by the personal benefit. If Trump was not motivated by the campaign contribution, but called off the investigation for legitimate public reasons, then he cannot be convicted of bribery.

So the argument that a court cannot inquire into the president’s motivations is spurious. And, indeed, Rivkin and Casey acknowledge that bribery is just a type of obstruction of justice, which means that the president can commit the crime of obstruction of justice just as he can commit the crime of bribery.

What is true is that a bribery conviction requires evidence of an independent act—the receipt of money—while obstruction of justice does not. This seems to be the key distinction that the president’s defenders rely on. They want to say that merely firing Comey cannot be obstruction of justice but engaging in some additional act (accepting a bribe, hiding evidence, etc.) would be. But why this distinction has legal significance has not yet been explained.