As the chairman of the House Oversight and Reform Committee, Elijah Cummings, who died last week at the age of 68, understood the necessity not to overplay your hand in politics. He also knew that impeachment is complicated. Indeed, questions regarding whether and how to pursue impeaching a president are not easy.

On the one hand, Donald Trump asked a foreign leader to investigate a political rival, setting off serious alarm bells regarding abuse of power. On the other hand, impeachment is an extraordinary constitutional remedy intended only for the gravest circumstances. A successful impeachment would remove a president elected by the people — the ultimate source of legitimacy in our democracy.


So how, then, should the Democrats, after Cummings, approach impeaching Mr. Trump?

First, the process must be fair both in appearance and substance. Ideally, an impeachment proceeding would be bipartisan. The alleged offense would be so clearly a crime — like ordering subordinates to commit burglary by breaking into the Democratic National Committee headquarters — that both sides would be unified in the effort.

That is not the case here. This is a partisan fight. The Democrats must therefore take every precaution to be fair and just. Any alternative would provide Mr. Trump helpful arguments in court and alienate large swaths of the electorate (many of whom are potential swing voters in next year's election). Given that over a dozen Republican Senators must cross the aisle in order for Mr. Trump to be removed from office, public opinion is paramount.

Second, there must be a clear violation of an existing crime. The Constitution allows for impeachment only for “high crimes and misdemeanors.” While there’s some ambiguity about what this means — and political calculations rightly play a role in the analysis — Congress must conscientiously respect the basic principles of criminal law given the gravity of the proceedings.

President Trump is, indeed, neither above the law (as he sometimes may think he is) nor below it (as some may want him to be).

First and foremost among the fundamental principles of criminal law is that an unambiguous law must be on the books at the time of the alleged offense. As the rule of lenity sets forth, any ambiguity regarding the scope of the law must be resolved in favor of the defendant. While the analysis will change as the investigation unfolds, current attempts to use campaign-finance regulations as a basis for establishing constitutional high crimes may not withstand subsequent judicial scrutiny.

Finally, the Democrats must remember that they are writing a rule book that will eventually apply to them, too. Every step the Democrats take to remove Mr. Trump from office can — and perhaps likely will — be used against them in the future. If a Democrat is elected president in 2020, congressional Republicans will be salivating at the prospect of impeachment. The more the Democrats push the limits now, the less ability they would have, then, to resist impeachment.

Impeaching the president of the United States is not easy. Nor should it be. The founders recognized this by requiring a two-thirds majority in the Senate in order to convict. If the Democrats overreach and ignore these guideposts, their impeachment effort could boomerang into a net negative. It could even, perhaps, tip the scales toward Mr. Trump in 2020.

William Cooper (wcooper11@gmail.com) is an attorney who has written for numerous publications including the Wall Street Journal, New York Daily News, Huffington Post and USA Today.