President Donald Trump says his impeachment trial should deliver on a goal he’s nurtured for months: unmasking the whistle-blower who started it all. But that would pose legal and ethical challenges that would be hard to overcome.

Federal laws promise anonymity for workers who step forward with alleged wrongdoing. In this case, a government employee — reportedly a CIA analyst — challenged the propriety of Trump’s request to Ukraine’s new president to investigate Joe Biden and other Democrats.

A Senate demand that the whistle-blower testify would probably be challenged in court as a violation of the law’s protections, and as a move that could put the unidentified person at risk while extracting only secondhand evidence of limited value. Lawmakers of both parties may share those concerns.