The House didn’t try hard enough to secure testimony.

Fearing a prolonged legal fight, House Democrats chose not to pursue the testimony of Mr. Bolton in court after he initially declined to cooperate voluntarily at the direction of the White House.

Republicans say that the House, as in most previous clashes over appearances by executive branch officials, should have sued to compel his testimony and exhausted its legal options to do so before moving ahead with articles of impeachment against Mr. Trump. In not doing so, they say, Democrats made a crucial error that they should not expect the Senate to correct.

But since the House approved the articles of impeachment, Mr. Bolton has indicated that he would testify if subpoenaed. Disclosures from a draft of his forthcoming book suggest that he has firsthand knowledge that would contradict Mr. Trump’s claim that he did nothing improper in withholding military aid from Ukraine.

Democrats say that for Republicans to willfully refuse to hear from Mr. Bolton after his change of heart amounts to an attempt to conceal Mr. Trump’s misconduct, particularly after complaining that the House’s charges against the president are built mainly on testimony from those without direct interaction with Mr. Trump.

House prosecutors claim they already have an ironclad case.

Republicans like to point out that the House impeachment managers, led by Representative Adam B. Schiff, Democrat of California, have repeatedly bragged that they developed an “ironclad” case against Mr. Trump, and compiled more than enough evidence to justify removing him from office for abuse of power and obstruction of Congress. If their case is so strong, Republicans argue, there should be no need to hear from witnesses.

Democrats do believe that their case is strong and that the reconstructed transcript of Mr. Trump’s July 25 phone call with President Volodymyr Zelensky of Ukraine, in which he asked for a “favor” of investigating the Biden family, is evidence enough. But witnesses could bolster their case and remove any lingering uncertainties about the president’s intentions. They also say the American public deserves a full account of the president’s conduct.