“We’re fighting all the subpoenas,” President Donald Trump declared in April, spurning congressional efforts to require administration officials to testify and provide documents to the committees in the House of Representatives that are investigating alleged presidential misconduct. In October, the White House announced it would not cooperate with any aspects of the House’s unfolding impeachment inquiry.

But just recently, Judge Ketanji Brown Jackson of the Federal District Court for the District of Columbia, ruled that former White House lawyer Donald McGahn must comply with a congressional subpoena and testify before House impeachment investigators. “Presidents,” Judge Jackson wrote, “are not kings,” adding, “they do not have subjects, bound by loyalty or blood, whose destiny they are entitled to control.”

Jackson’s ruling, already appealed by the government, has framed a critical constitutional issue: Must the executive branch comply with subpoenas issued by the legislative branch? The answer to this question, in turn, rests on matters fundamental to our system of government in the United States, namely the relative powers of the legislative and executive branches, established in our Constitution as co-equal branches of government.

Stated simply, can Congress force the executive branch to testify before Congress or can the president prevent his aides from testifying by claiming “executive privilege”? This question will be contested fiercely in the courts over the next several months, and is likely to land at the Supreme Court.

But wait a minute. How did this problem arise? What are the president and the House arguing about? And why is it so important?

Let’s first look at the origins of this clash between President Trump and the House of Representatives. Different House committees, exercising their constitutionally based power to investigate, have begun collecting evidence – by questioning witnesses and reviewing documents – in an impeachment inquiry focused on whether the president pressured the president of Ukraine to open a corruption investigation of former Vice President Biden, a leading contender for the 2020 Democratic presidential nomination, and his son. As part of their investigation, the House committees need to question witnesses and review documents that would shed light on the issues in question. To obtain this information, the committees have issued subpoenas, which are official documents ordering a person to appear to provide testimony or to produce documents.

Many of the relevant witnesses, however, are either current or former officials in the executive branch and that same branch maintains most of the important documents. President Trump, as head of the executive branch, has ordered his aides to ignore congressional subpoenas, claiming “executive privilege” and asserting that administration officials have “absolute testimonial immunity” and thus do not have to testify before Congress. While some people – including Dr. Fiona Hill and Ambassador Gordon Sondland, among others – have defied the president’s orders, many administration officials have complied, thereby thwarting the House committees’ efforts to gather information relevant to the impeachment inquiry.

Why is this clash so important?

On one level, the clash between President Trump and the House committees reflects the predictable adversarial tactics of the impeachment inquiry, which pits the Democratic-majority House against a Republican president in a confrontation where each wants to prevail. That is, House committees want a robust investigation into the president’s alleged abuse of his office, and the president aims to protect himself by resisting inquiries into his purported misdeeds.

But on another, more fundamental level, the clash amounts to a showdown between the executive and legislative branches of government. On the one hand, Congress is constitutionally permitted to investigate the conduct of a president and officials in the executive branch, including undertaking to impeach a president. On the other hand, executive privilege, which shields from compelled disclosure certain types of confidential executive branch communications, is a recognized legal doctrine.

But can President Trump use executive privilege to resist all congressional subpoenas and simply order his aides to refuse to testify?

In United States v. Nixon, decided in 1974, the Supreme Court acknowledged there was a limited executive privilege in areas of military or diplomatic affairs. But the court rejected both the claim that the privilege was absolute and the assertion that all internal executive branch communications were protected from compelled disclosure. That ruling meant Nixon had to release the tapes of incriminating discussions in the Oval Office, leading to his resignation just weeks after the court’s opinion. Other courts have ruled that the “absolute testimonial immunity” claim, which would permit executive branch officials to refuse to comply with congressional subpoenas, is invalid.

So back to Judge Jackson’s ruling issued last month. While her opinion is clear and strongly worded, the government has already said it would appeal, and the case is likely to end up at the Supreme Court. The court has a different and more conservative make-up today than it did in 1974 when United States v. Nixon was decided and thus, although precedent indicates executive branch officials eventually will be required to testify, the judicial outcome remains uncertain.

In the meantime, however, the legal battle and the standoff with Congress assuredly complicate President Trump’s re-election prospects.

(Jill Blackmer, a retired lawyer, teaches at St. Paul’s School.)