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Staring impeachment in the eye, President Donald Trump has opted for denial, moving to obstruct House Democrats and queuing up a constitutional crisis. With two branches of government preparing for political war, the question now is whether the third -- the courts -- will be dragged into the fray.

The US system, as your history teacher taught you , is made of three coequal branches of government. When one of them stops listening to the others, there's a breakdown.

And, not liking what he's hearing from a Democratic-controlled House of Representatives after he invited foreign powers to investigate his 2020 rival Joe Biden, Trump has essentially stopped listening to Congress.

Democrats push quickly forward

But far from using the third branch to put him in check, Speaker Nancy Pelosi and other House Democrats are plunging forward with their impeachment inquiry, issuing subpoenas they now know won't be heeded. Pelosi does not yet want to seek help from the courts, where things might slow down or, worse yet for Democrats, a conservative majority on the Supreme Court could rewrite the definition of presidential power.

Ultimately, the question for Democrats is whether they need a series of hearings, fact-finding and documents in order to accuse Trump of high crimes and misdemeanors, the squishy constitutional requirement for impeachment.

Trump has periodically stress-tested the Constitution, as when he tried to close US borders to Muslims, when he shut down parts of the government over his border wall, when he used a national emergency law to divert money from the Pentagon to the wall and when he tried repeatedly to squash the special counsel investigation into possible collusion between his 2016 campaign and Russia. He distrusts the bureaucracy he leads and resents the branches of government that check his power.

But now, with a whistleblower from the bureaucracy nudging Congress toward impeachment, his presidency is actually in peril.

Plenty of Democratic voters and officials on the left think they've already got the goods for impeachment in the transcript of a July call with Ukrainian President Volodymyr Zelensky in which Trump badgered the foreign power to look into his domestic rival and tested US law that forbids candidates from seeking anything of value from foreign powers. If that wasn't enough, Trump doubled down when he openly invited China to go after Biden.

Now he's refusing to cooperate with Congress as it tries to execute its clearly defined role of oversight and impeachment.

Why bother asking Supreme Court Chief Justice John Roberts to weigh in on all that?

"Richard Nixon was held accountable by the House Judiciary Committee for doing exactly what Donald Trump is doing now," the presidential historian Tim Naftali said Wednesday on CNN, referring to Trump's tactic of stonewalling Congress.

Not bending over for bipartisanship

But he also said there's great peril for Democrats in not ticking the boxes of bipartisanship. Democratic voters and lawmakers do not equal a majority of the country or the Senate.

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"Stop making it the Democrats versus Trump. How about Americans for the rule of law against a lawless presidency?" Naftali said, urging Pelosi to hold a constitutionally unnecessary but perhaps politically savvy vote to authorize the impeachment inquiry in which Democrats are already fully engaged. "I don't understand why the Democrats are not working with some Republicans. There must be some Republicans in the House of Representatives who are concerned about authoritarianism."

To be sure. But they might come to reject the leader of their party, who holds incredible sway with their political base, only as a last resort -- particularly since the longer this takes, the closer it gets to Election Day 2020.

"In terms of constitutional history, the argument this is invalid is wrong and will be tossed out by some court at some point," said Naftali. But it might not get to the courts if Pelosi and her lieutenants use Trump's stonewalling as evidence of impeachable offenses rather than try it in the courts.

That means the executive branch is actively trying to squash the legislative branch and the judicial branch might not even weigh in.

It all sets the stage for a constitutional crisis, according to Keith Whittington, a Princeton political science professor.

"The President has thrown down the gauntlet, dared Congress to impeach and hold him accountable for the claims, and even Republicans in Congress have to think seriously about what the long-term consequences are, allowing a President to make those kinds of claims and stick by them without consequences," he said.

A political fight, not a legal one

But Trump isn't exactly making legal arguments in his own defense. The White House issued a bombastic letter rejecting the impeachment inquiry Tuesday. It included legal citations, but it would be thrown out by any court in the country, according to Jennifer Rodgers, a CNN legal analyst and lecturer at Columbia Law School.

"The name of this game is delay, delay, delay, with the election coming," she said on CNN, predicting an impeachment vote by the end of the year -- ambitious Democrats say Thanksgiving -- regardless of whether the White House complies with any of the growing body of subpoenas being thrown at it.

"I think increasingly the administration is in the territory where really it's making political arguments, not really legal arguments," said Whittington. "They're not arguments that are designed to persuade lawyers that there's some kind of decent constitutional grounds for what the administration wants to do here. It's designed to set up a political fight, including a political fight in the Senate."

On the phone with McConnell

Trump has rejected the impeachment inquiry as a "kangaroo court," but that doesn't mean it hasn't caught his attention. He's talking to Senate Majority Leader Mitch McConnell, his would-be protector in the Senate, up to three times each day, according to a new CNN report . It takes only a bare majority of Democrats to impeach Trump in the House, but 20 Republicans would have to vote in the Senate to remove him from office.

Despite some rumblings from the likes of Sens. Mitt Romney of Utah, Rob Portman of Ohio, Susan Collins of Maine and Ben Sasse of Nebraska, those 20 votes seem a long way off.

There's no indication yet that the impeachment effort will push Trump out of office, but there's every indication it is going to be a historical test for the Constitution.