McQuade is a professor at the University of Michigan law school and was the U.S. attorney in Detroit during the Obama administration.

The legal and constitutional battles sparked by President Trump's behavior could affect how the federal government works for generations, long after the impeachment trial is over.

After the last Senate staffer turns out the lights, major questions remain to be decided outside of the Capitol about the limits of presidential power, the willingness of courts to decide political questions and the ability of Congress to exercise effective oversight and hold a president accountable.

Here are three of those questions.

What are the limits of presidential power?

The aggressive exercise of executive power by Trump has put this power under court scrutiny.

Trump's vow to "fight all the subpoenas" breaks from the traditional process – negotiation and accommodation – that previous presidents have used to resolve disputes between branches of the government.

As a result, several cases are currently pending, including a legal challenge brought by the House Judiciary Committee to compel the testimony of Don McGahn, Trump's former White House counsel. The House had sought McGahn's testimony about Trump's alleged obstruction of justice in the investigation of special counsel Robert Mueller into Russian election interference.

McGahn challenged the subpoena issued by the Judiciary Committee on the grounds of absolute immunity, arguing that he – a close aide to the president, and a member of the co-equal executive branch – need not appear before Congress to answer questions at all.

U.S. District Judge Ketanji Brown Jackson rejected this argument, saying that while McGahn could possibly assert executive privilege about individual questions, he could not refuse to appear altogether.

Executive privilege is not specified in the Constitution. But the Supreme Court has recognized that a president may shield from disclosure certain sensitive information and communications to encourage candid advice from aides and to protect national security and other sensitive information.

"However busy or essential a presidential aide might be, and whatever their proximity to sensitive domestic and national-security projects, the President does not have the power to excuse him or her from taking an action that the law requires," Judge Jackson wrote.

The case is now on appeal, and during oral argument in early January, the committee's lawyer said that additional impeachment articles could be filed based on McGahn's testimony.

In 1974, in United States v. Nixon, however, the court stated that the privilege is not absolute, and must yield in some circumstances, such as a criminal investigation. Absolute immunity, which courts have not recognized, goes even further than executive privilege, permitting an aide to refuse to appear altogether.

Regardless of the outcome of the case, a court decision in the McGahn case will provide clarity that will weaken or strengthen the negotiating position of future presidents.

Should courts step into political conflicts?

Some of the cases still pending could determine how much power courts have in impeachment matters.

Under what is known as the "political question doctrine," courts typically avoid what are known as "political questions"that involve branches of government in conflict. They have dismissed most cases that present such questions, deferring to the other branches to resolve them. In the more than 200 years between 1789 and 2017, when Trump took office, courts heard only five cases for presidential claims of executive privilege in response to a congressional subpoena.

In the 1993 case of Nixon v. United States (no, not that Nixon, U.S. District Judge Walter Nixon), the Supreme Court held that a federal judge could not appeal to a court seeking to overturn his conviction at a Senate impeachment trial. The Constitution, the court ruled, gives the Senate the sole power to try all impeachments.

Concurring opinions in the Nixon case, however, left open the possibility of an appeal to courts for an impeachment trial that was conducted "arbitrarily," that is, lacking procedural fairness.

Trump's personal lawyer, Rudolph Giuliani, suggested at one time that Trump file a court challenge to dismiss the articles of impeachment.

While that seems unlikely in light of the Nixon case, the political question doctrine is likely to figure in other pending cases, such the effort by Congress to seek grand jury material from Mueller's investigation.

During oral argument earlier this month in the case over grand jury material pending before the court of appeals, one of the judges expressed reluctance to decide the case because it involves a political question.

As the courts decide the cases involving McGahn's testimony, the Mueller grand jury material, and any challenge arising from Trump's impeachment trial, the contours of the political question doctrine will become more defined.

Will the executive, legislative and judicial branches collide?

In the impeachment's aftermath, the extent of Congress' ability to serve as a valid check on presidential power will become more clear.

The framers of the Constitution envisioned a Congress that would provide oversight over a president. They did not count on members of Congress having more loyalty to their party than to their institution.

If the Senate were to acquit the president in the face of additional incriminating evidence, the institution's ability to serve as a credible check on future presidents could be damaged.

The impeachment trial itself could cause all three branches to collide. Former national security adviser John Bolton has publicly stated that he would testify if subpoenaed by the Senate. Trump has said he would he would invoke executive privilege to block Bolton's testimony.

If the Senate wanted to compel the testimony, the presiding Chief Justice John Roberts would decide the standoff between the president and the Senate. If he were to rule in favor of the Senate and order Bolton to testify, could President Trump appeal that decision to the Supreme Court? Would the Court be willing to decide such a political question about impeachment? Would the Senate arrest and jail a witness for refusing to testify?

There are no rules for what happens then.

Throughout his presidency, Trump has been a disrupter of normal procedures. It appears that he will continue that trend even after impeachment.

This article is republished from The Conversation under a Creative Commons license. Click here to read the original article.