Beyond the politics and partisanship of the moment, President Donald Trump and majority Democrats in the House of Representatives are colliding in the most extraordinary test of the Constitution's separation of powers in many years.

"We are heading rapidly towards a constitutional crisis," David Rothkopf, a political scientist and specialist in international relations, told MSNBC. This is because Trump doesn't accept the legitimacy of Congress's bid to impeach him as sought by House Democrats, and the House under those Democrats won't back away from efforts to punish him for what critics call abuses of power.

"It's headed, I think, for the Supreme Court," says Rutgers University political scientist Ross Baker.

Trump "is shaking the foundations of the Republic," Kerry Kircher, former House counsel for the Republicans, told The Washington Post.

Former Vice President Joe Biden, a leading candidate for the Democratic presidential nomination in 2020, told a crowd in New Hampshire on Wednesday that Trump should be impeached for "shooting holes in the Constitution." Trump has accused Biden of corruption in Ukraine but those allegations are unsubstantiated. Many Democrats argue that Trump's charges against Biden are so improper that they have become part of the presidential impeachment inquiry.

White House counsel Pat Cipollone expressed and expanded Trump's defiance Tuesday when he told Democratic congressional leaders that the White House will not participate in the House's ongoing impeachment inquiry. In a letter to congressional leaders, Cipollone declared that the inquiry "lacks any legitimate constitutional foundation" and is an effort to "nullify the outcome of the democratic process" by changing the result of the 2016 election, which Trump won, and influencing the 2020 election.

Cipollone also wrote, "Given that your inquiry lacks any legitimate constitutional foundation, any pretense of fairness, or even the most elementary due process protections, the executive branch cannot be expected to participate in it. Because participating in this inquiry under the current unconstitutional posture would inflict lasting institutional harm on the executive branch and lasting damage to the separation of powers, you have left the president no choice."

House Speaker Nancy Pelosi argued that Trump is digging himself into deeper trouble by obstructing the House inquiry and, in a statement, told Trump, "You will be held accountable." The California Democrat also disputed the contention of Trump and his allies that the House should vote formally to conduct the impeachment inquiry or the probe won't be legitimate. Pelosi replied that such a formal vote isn't necessary at this stage and isn't required by law or the Constitution.

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Democratic leaders have been escalating their confrontation with the White House by subpoenaing a stream of documents and demanding testimony from key administration officials. Trump is refusing to cooperate, which the Democrats see as obstruction. This refusal could become one of several articles of impeachment. The Cipollone letter amounted to an all-out counteroffensive.

But political scientist Baker says it is "less a legal defense than a political barrage." And many legal scholars say House Democrats have the superior legal argument. A survey of 13 of those scholars by Vox, a news and opinion website, found strong support for the House leaders' arguments and scant support, and sometimes outright scorn, for Trump's.

"The letter from the White House is a political stunt that misinterprets the Constitution, ignores relevant precedents and defies common sense," Lisa Kern Griffin, a law professor at Duke University, told Vox. "The Constitution does not say much about impeachment, but what it does state is clear, simple, and right there in Article I. The House 'shall have the sole Power of Impeachment' and the Senate 'shall have the sole Power to try all Impeachments.'"

Diane Marie Amann, a law professor at the University of Georgia, told Vox, "The White House has taken a political position that assumes almost unlimited executive power and pays little heed to the checks and balances upon which the US Constitution is founded."

Ilya Somin, law professor at George Mason University, told Vox, "The impeachment power belongs to the House. It applies in situations where there is reason to believe the president has committed 'treason, bribery or other high crimes and misdemeanors.' The founders drafted the impeachment clause to cover a wide range of abuses of power, including ones where there is no violation of criminal law. If Trump withheld aid from Ukraine in an attempt to pressure them into investigating a political opponent, he likely both violated the Constitution and committed a crime."

Rothkopf told MSNBC that the Constitution doesn't give the executive branch the right to decide whether or not to impeach.

"The Constitution gives that to the Congress," he said.

Constitutional law professor Michael Gerhardt of the University of North Carolina agreed.

"The Constitution says the House has the sole power to impeach," he told CNN. Baker says the impeachment power was given to Congress by the framers as a tool to "rein in an out-of-control executive. Trump is attacking that."

Rothkopf, who formerly worked in the Commerce Department of Democratic President Bill Clinton, added: "Somewhere this is going to have to be tested – in a court, by the Congress taking an action by putting somebody in jail as a consequence of it," or in some other way.

Constitutional law expert Laurence Tribe, a professor at Harvard Law School, told MSNBC, "The key point isn't just that it's a federal felony to solicit foreign help in an American election. That sounds technical. The point is that it wasn't just soliciting foreign help, it was leveraging the powers that go with the office to intervene in our policy, whether it's with tariffs and trade with China or helping the little Ukrainian defense against the powerful Russian bear. It is that matter, of taking the power of the presidency and using it for selfish purposes, that is the core impeachable offense. And that could be a central article of impeachment."

Tribe also said Cipollone's legal analysis is "vacuous" and "irrelevant" to the current impeachment debate.

Tribe predicted that the courts will reject Trump's arguments, even judges whom Trump appointed.

"We have a lawless president who is in it for his own benefit," the professor said.

The final arbiter may be the Supreme Court. And it's unclear how the conservative majority there, including two new conservative justices named by Trump, would define congressional power vs. executive power.

Beyond the legalities, Trump may be attempting to delegitimize the impeachment process in order to stir up his base for the 2020 election. He regularly condemns the "deep state" of entrenched power brokers in Washington, the Democrats and the "fake news media" for trying to destroy his presidency from the beginning. Now he can argue that impeachment is part of such an unfair, vindictive anti-Trump campaign.

The main focus in Congress and the media has been a July 25 phone call in which Trump sought a "favor" from Ukrainian President Volodymyr Zelenskiy. This favor, according to a federal whistleblower and a partial transcript of the conversation released by the White House, was having Ukraine pursue a corruption investigation of former Vice President Biden and Biden's son Hunter, who had business dealings in Ukraine. Trump said he did nothing wrong.

But public opinion appears to be turning against the president. A Washington Post-Schar School poll released this week found that 58 percent of Americans say the House was correct to start the impeachment inquiry and 49 percent of Americans want the House to impeach Trump and call for his removal from office. Thirty-eight percent oppose the inquiry. The findings represent a deterioration in support for the president from a few weeks ago.

More than 8 in 10 Democrats support the inquiry and nearly 8 in 10 favor a House vote to recommend that Trump be removed. About 7 in 10 Republicans oppose the inquiry and nearly 3 in 10 support it. One-fifth of Republicans favor a House vote recommending the president's removal. Among independents, 57 percent support the impeachment inquiry and 49 percent say the House should recommend removing Trump.

Impeachment proceedings have occurred three times in U.S. history. In 1998, the House, then controlled by Republicans, impeached Bill Clinton for perjury and obstruction of justice after he lied under oath about having an affair with former White House intern Monica Lewinsky. The GOP-controlled Senate acquitted Clinton in early 1999. Many members of Congress and many Americans felt that Clinton was impeached for personal failings that had nothing to do with his policies, his public role as president or violating the separation of powers. His job-approval rating climbed after the impeachment furor subsided.

There are more similarities to the Trump situation in the impeachment crises of President Andrew Johnson in 1868 and President Richard Nixon in 1974. Both of those cases focused on whether the chief executive overstepped his constitutional powers and abused his authority.

Johnson, who succeeded to the presidency after Abraham Lincoln was assassinated in 1865, got into a bitter confrontation with "radical Republicans" in Congress. He disagreed with their hard-line policies toward the defeated Confederate states after the Civil War and the Republicans' desire to protect the freedom and rights of former slaves. Johnson's impeachment was based on a fundamental dispute over an easy-to-understand question: Could the president legally remove a Cabinet member without congressional approval? The House concluded that he needed congressional approval but Johnson resisted, insisting he could fire Cabinet members as he pleased. After his impeachment by the House, Johnson barely survived the acquittal vote in the Senate. Years later, the Supreme Court supported Johnson's position on the Cabinet.

Perhaps the best parallel is to Richard Nixon. The Democratic-controlled House and Senate moved separately to investigate him for official misconduct, including obstruction of justice and a cover-up of the Watergate abuse-of-power scandal. Nixon stonewalled Congress in many ways and the courts ruled against him several times. He resigned as the House was moving to impeach him in August 1974.

In another case that is relevant but is rarely mentioned today, during the 1830s, Andrew Jackson refused to obey a ruling by the Supreme Court to respect the rights of native Americans on the land where they lived. Instead, Jackson forced them off tribal land in the Southern states. He also got into other tangles with Congress, such as a dispute over the central bank, which Jackson eventually destroyed. Jackson was such a powerful figure that he was never impeached, but he was censured by the Senate in a separate dispute during 1834. President Trump has listed Jackson as one of his political heroes partly because of Jackson's decisiveness and bold leadership.