On Thursday, the U.S. Supreme Court will hear Donald Trump’s arguments that he is immune from prosecution for his efforts to steal the 2020 presidential election. It is likely that all — or nearly all — of the justices agree that a former president who attempted to seize power and remain in office illegally can be prosecuted. I suspect that some justices may also wish to clarify whether doctrines of presidential immunity might apply in other contexts — for example, to a president’s actions as commander in chief during a time of war. But the justices should also recognize the profoundly negative impact they may have if the court does not resolve these issues quickly and decisively.

If delay prevents this Trump case from being tried this year, the public may never hear critical and historic evidence developed before the grand jury, and our system may never hold the man most responsible for Jan. 6 to account.

The Jan. 6 House select committee’s hearings and final report in 2022 relied on testimony given by dozens of Republicans — including many who worked closely with Mr. Trump in the White House, in his Justice Department and on his 2020 presidential campaign. Special counsel Jack Smith’s election-related indictment of Mr. Trump relies on many of the same firsthand witnesses. Although the special counsel reached a number of the same conclusions as the select committee, the indictment is predicated on a separate and independent investigation. Evidence was developed and presented to a grand jury sitting in Washington, D.C.

The indictment and public reporting suggest that the special counsel was able to obtain key evidence our committee did not have. For example, it appears that the grand jury received evidence from witnesses such as Mark Meadows, the former Trump chief of staff, and Dan Scavino, a former Trump aide, both of whom refused to testify in our investigation. Public reporting also suggests that members of Mr. Trump’s Office of White House Counsel and other White House aides testified in full, without any limitations based on executive privilege, as did Vice President Mike Pence and his counsel.

The special counsel’s indictment lays out Mr. Trump’s detailed plan to overturn the 2020 election, including the corrupt use of fraudulent slates of electors in several states. According to the indictment, senior advisers in the White House, Justice Department and elsewhere repeatedly warned that Mr. Trump’s claims of election fraud were false and that his plans for Jan. 6 were illegal. Mr. Trump chose to ignore those warnings. (Remember what the White House lawyer Eric Herschmann told Mr. Trump’s alleged co-conspirator John Eastman on Jan. 7, 2021: “Get a great f’ing criminal defense lawyer. You’re gonna need it.”) There is little doubt that Mr. Trump’s closest advisers also gave the federal grand jury minute-to-minute accounts of his malicious conduct on Jan. 6, describing how they repeatedly begged the president to instruct the violent rioters to leave our Capitol and how Mr. Trump refused for several hours to do so as he watched the attack on television. This historic testimony about a former president’s conduct is likely to remain secret until the special counsel presents his case at trial.

As a criminal defendant, Mr. Trump has long had access to federal grand jury material relating to his Jan. 6 indictment and to all the testimony obtained by our select committee. He knows what all these witnesses have said under oath and understands the risks he faces at trial. That’s why he is doing everything possible to try to delay his Jan. 6 federal criminal trial until after the November election. If the trial is delayed past this fall and Mr. Trump wins re-election, he will surely fire the special counsel, order his Justice Department to drop all Jan. 6 cases and try to prevent key grand jury testimony from ever seeing the light of day.