SENATORS FACE a historic decision Friday: whether to shut down the trial of President Trump without hearing what they know would be essential evidence. Mr. Trump has denied for months that he withheld military aid and a White House meeting from Ukraine’s president in an effort to force politicized investigations, including of former vice president Joe Biden. The president’s lawyers have insisted that there is no firsthand testimony to the contrary. Yet, now, senators know that former national security adviser John Bolton can supply that testimony and that he is prepared to appear if called.

Polls show that Americans overwhelmingly want the senators to hear witnesses. There is no precedent for an impeachment trial without testimony. If the Senate refuses, it will ratify a coverup, making any acquittal of Mr. Trump meaningless.

Mr. Bolton is not the only witness who could enlighten the Senate, and the country, about the Ukraine affair. Acting White House chief of staff Mick Mulvaney also has direct knowledge of Mr. Trump’s intention in withholding the Ukraine aid. Vice President Pence knows why Mr. Trump suddenly ordered him not to attend the inauguration of Ukrainian President Volodymyr Zelensky last May. Rudolph W. Giuliani, the president’s personal lawyer, and his former sidekicks Lev Parnas and Igor Fruman know in detail how Mr. Zelensky and his top aides were pressured.

Republicans argue that an attempt to subpoena Mr. Bolton or other witnesses would lead to court battles with the White House — even as they fault the House for not undertaking the same fight. One of the president’s lawyers has advanced the extraordinary defense that Mr. Trump was entitled to trade official acts for help with his reelection — a position, that, if ratified, would grant Mr. Trump and future presidents vast powers to rig elections.

GOP leaders vow that if the Senate votes to call witnesses, they will move to summon Mr. Biden, his son Hunter and others. That would be shameful: By forcing the Bidens to testify, senators would be handing Mr. Trump what he was seeking all along, the chance to sow bogus questions about the probity of a leading Democratic presidential candidate. Democrats, however, should not be intimidated by this threat into backing away from the demand that Mr. Bolton and other relevant witnesses appear.

Ironically, senators who support a coverup now will only limit their acceptable options as the trial ends. Serious arguments have been made against conviction. One is that, because the president’s extortion scheme never bore fruit — no investigations were announced, and U.S. aid was eventually delivered — removal from office is an excessive remedy. Another is that a verdict on Mr. Trump’s tenure should be rendered by voters nine months from now.

But if Republicans muzzle potential witnesses and suppress relevant documents, senators who respect the Constitution will be left with only one honorable choice. Mr. Trump tried to use his office to force Ukraine to intervene in the 2020 election. He and his lawyers have not only refused to admit wrongdoing but have brazenly asserted the president’s right to engage in such manipulations. If that proposition is the one to be decided, and what should be a full and fair trial is aborted on the president’s orders, the only justifiable vote will be for conviction.

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