The impeachment of a president of the United States is an immensely powerful constitutional act. Donald Trump is only the third president in more than 230 years to face trial in the Senate after being impeached by the House of Representatives. The trial that begins in earnest next week could result in Mr Trump becoming the first president to be dismissed from office. His two impeached predecessors – Andrew Johnson in 1868 and Bill Clinton in 1999 – each escaped that outcome. Mr Trump is likely to escape it too. Nevertheless, this is a hugely solemn moment for the republic. Most Americans rightly take it seriously.

It is easy, but wrong, to overlook this. Mr Trump treats the impeachment and trial with everything bar solemnity. He regards it as nothing more than a partisan witch-hunt. Like Mr Clinton he will not attend the trial in person. Unlike Mr Clinton he is likely to spend the next month raging against the process and insulting his accusers. But this does not mean the trial itself is trivial. The formality of the initial proceedings this week is appropriate. Even in polarised and partisan times, the American public should not dismiss it as trivial either.

The two charges – the articles of impeachment – against Mr Trump are genuinely serious. The first says he “abused the powers of the presidency” by trying to get Ukraine to intervene in the 2020 election. He did this in three ways, the article states: by corruptly soliciting investigations by Ukraine that would assist his re-election; by corruptly withholding aid to Ukraine unless it cooperated; and by corruptly persisting with pressuring Ukraine afterwards for his own benefit. The article concludes that this makes Mr Trump a threat to US national security if he remains in office. All in all, it is very much the kind of conduct that the Congress’s power to impeach was designed to control.

The second article charges Mr Trump with obstruction of Congress as it attempted to carry out its lawful power of impeachment. It says the obstruction involved defiance of subpoenas, and orders by Mr Trump to government officials not to comply either. Such defiance of an impeachment process is, in principle, itself impeachable. It will be tested during the trial when the Senate votes on whether to call officials, such as the former national security adviser John Bolton, the acting White House chief of staff Mick Mulvaney and the Soviet-born US businessman Lev Parnas, as witnesses.

This will be a pivotal moment for America. If the Senate does not call the witnesses, it is in effect saying that Mr Trump is above the law. That would be an extraordinary abdication of constitutional responsibility. The US upper house was originally created as a restraint on both the elected house and the president. It was there, the historian Robert Caro writes, as a check on “the possibilities for tyranny inherent in executive authority”. That is why the Senate has so many powers, including that of trying an impeachment. Yet the current Senate is a place of ruthless partisanship and irreconcilability, acting invariably on behalf of Mr Trump.

This trial is therefore not just a defining moment for Mr Trump and for the presidency. It is also a defining moment for the Senate itself. If the Senate votes on purely party lines to acquit and Mr Trump is then re-elected, this would be an infamous outcome. The idea that a president can abuse his office for personal gain would be accepted. A new and lawless presidential legitimacy would be sanctioned. And the checks and balances of the US constitution would no longer exist and would need to be built afresh.