HOUSE SPEAKER Nancy Pelosi (D-Calif.) on Thursday officially instructed key committee chairs to draft articles of impeachment against President Trump. But it’s still unclear what those articles would cover: just the Ukraine scandal, or other instances of presidential misconduct, particularly relating to the Russia investigation?

The Democrats would be wise to focus narrowly on Ukraine.

Focused impeachment articles would not detract from the power of the charges against Mr. Trump. They would be more persuasive to the many Americans who hesitated to support impeachment before the Ukraine affair emerged — a group that includes Ms. Pelosi and many other top Democrats. It also includes those who, in polling, swung from opposing to supporting impeachment, a swing that flipped public opinion.

There are substantive reasons for their previous hesitation. Former special counsel Robert S. Mueller III, who ran the Russia investigation, by no means exonerated Mr. Trump. But neither did he uncover grave subversion of justice. Rather, Mr. Mueller arguably found instances of attempted obstruction of justice. Though he fired FBI Director James B. Comey, the president watched as Mr. Mueller later forced national security adviser Michael Flynn to take a plea bargain. Mr. Trump could have fired Mr. Mueller after then-White House Counsel Donald McGahn did not. But the president restrained his impulse and allowed, however grudgingly and intemperately, Mr. Mueller’s investigation to proceed to its conclusion. The executive branch also largely cooperated with the special counsel probe, which it has not done with Congress’s Ukraine investigation.

George Washington University law professor Jonathan Turley on Wednesday reasonably cautioned the House Judiciary Committee that Congress should avoid establishing the precedent that every instance of presidential bad behavior — even very bad behavior — warrants impeachment. The next time a Democratic president does something sleazy, Democrats should be able to argue that they respected the norm that Congress impeaches presidents only in exceptional circumstances — and that there are other ways Congress can punish a president. Given the potentially disastrous consequences of ever-more-frequent impeachment efforts, this is a norm worth respecting.

By the same token, the current impeachment process enjoys a legitimacy that it would not otherwise have if Democrats had jumped to impeach based on Mr. Mueller’s findings alone. That Ms. Pelosi resisted impeachment for so long lends credibility to the argument that Mr. Trump’s conduct toward Ukraine was not just very bad but unique and unacceptable.

It would also be strange for the Mueller findings to appear in the forthcoming impeachment articles because the Russia investigation has hardly featured in the weeks-long impeachment inquiry, which has systematically laid the foundation for a different indictment of the president.

Americans — voters — need not blind themselves to Mr. Trump’s past misdeeds. Nor should they ignore the connections between the Russia investigation and the Ukraine affair. For example: One of Mr. Trump’s key demands from Ukrainian President Volodymyr Zelensky was to substantiate a nonsense claim that Ukraine, not Russia, was behind the 2016 election interference from which Mr. Trump benefited. But Democrats should make their cleanest, clearest case that Mr. Trump has transgressed the ultimate line. It is on them to convince the public not just that the president could be impeached but also that he must be.

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