IN PRESSURING Ukraine’s president to announce an investigation of Joe Biden and his son, President Trump was seeking to tarnish a possible opponent in the 2020 election by creating the false impression that he was guilty of corruption. Now Mr. Trump and his supporters on the House Intelligence Committee evidently hope to use the public impeachment hearings, which open Wednesday, to accomplish the same end.

“Biden’s actions . . . are certainly looking very corrupt (to put it mildly!) to me,” Mr. Trump tweeted Tuesday. “Both Bidens should be forced to testify in this No Due Process Scam!”

The allegations Mr. Trump was referring to — that Mr. Biden pressed for the removal of a public prosecutor to benefit his son Hunter — have been comprehensively debunked, including in sworn testimony by the Trump administration’s own former envoy to Ukraine. But don’t expect that to stop Mr. Trump or his most slavish followers. They will do their best to turn what should be an examination of Mr. Trump’s abuse of office into a televised orgy of mudslinging at Mr. Biden and at the whistleblower who called attention to the president’s misdeeds.

Committee Chairman Adam B. Schiff (D-Calif.) has vowed to prevent such distortion of the process. Republican requests for testimony about the Bidens or Ukraine’s supposed interference in the 2016 presidential election — another debunked conspiracy theory promoted by Mr. Trump — will rightly be denied, and Mr. Schiff hinted in a memorandum released Tuesday that committee members who try to disclose the identity of the confidential whistleblower could be subject to action by the House Ethics Committee.

Republicans of course have a right to defend Mr. Trump. But the responsible way to do so is to focus on the evidence for and against the inquiry’s central questions: whether Mr. Trump sought to leverage U.S. aid and the prospect of an Oval Office meeting to induce Ukrainian President Volodymyr Zelensky to open the political investigations Mr. Trump wanted, and whether the White House tried to cover up that conduct.

At least some Republicans appear ready to engage with the charges. A memo prepared by GOP committee staff that Axios published Tuesday outlines four responses, including that a rough transcript of a July 25 phone call between Mr. Trump and Mr. Zelensky “shows no conditionality or evidence of pressure”; that both presidents have denied such pressure; that the Ukrainian government did not know U.S. military aid had been withheld at the time of the call; and that the aid ultimately flowed to Ukraine.

The hearings are likely to prove challenging to this defense, since testimony will make clear that the campaign to push Ukraine into investigating the Bidens was not limited to the July 25 phone call but extended over months and included multiple messages from envoys spelling out a trade-off between Mr. Zelensky’s announcement of the probes and the White House meeting he wanted, as well as military aid. As for the phone call, at least one witness who listened to it, Army Lt. Col. Alexander Vindman, will likely testify that he found it so disturbing that he immediately reported it to White House lawyers.

The sturdiest Republican defense might be that suggested by Sen. Rob Portman (R-Ohio) and Rep. Mac Thornberry (R-Tex.), among others: that what Mr. Trump did was wrong but does not merit impeachment. Yet that argument is rejected by Mr. Trump, who insists that Republicans defend his actions as “perfect.” If they stick to that script, the impeachment hearings will only discredit them.

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