Democrats say Trump has led us into a 'constitutional crisis.' Republicans disagree. Here's what it means.

• Democrats and Republicans are debating over whether the US is in a constitutional crisis amid the ongoing, dramatic dispute over the Mueller report.
• The phrase "constitutional crisis" is overused, and has multiple interpretations.
• "Constitutional crises arise out of the failure, or strong risk of failure, of a constitution to perform its central functions," according to political scientist Keith Whittington.
• Political scientists Julie Azari and Seth Masket say there are four types of constitutional crises: When the Constitution doesn't tell us what to do in a given situation; when the meaning of the Constitution is unclear and comes into question; when the Constitution provides guidelines on what to do, but following through would be impractical or politically unfeasible; when the government's institutions and the Constitution's system of checks and balances fail.
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House Judiciary Chairman Rep. Jerry Nadler on Wednesday said the US is in a "constitutional crisis" because the White House has vowed to resist subpoenas related to the special counsel Robert Mueller's investigation into Russia's interference in the 2016 US election.

After the House Judiciary Committee in a rare move on Wednesday voted to hold Attorney General William Barr in contempt for not complying with a subpoena for a full, unredacted version of Mueller's report on the Russia probe, Nadler said, "We are now in a constitutional crisis."

"Now is the time of testing whether we can keep our republic, or whether this republic is destined to change into a different, more tyrannical form of government," Nadler added. "We must resist this."

Meanwhile, Republicans like Sen. Lindsey Graham of South Carolina are scoffing at Nadler's assertion.

After the contempt vote on Wednesday, Graham tweeted, "What is going on in the House Judiciary Committee is not a constitutional crisis. It is a political stunt!"

Read more: Being held in contempt of Congress is one of the most severe punishments on Capitol Hill. Here's what such a move could mean.

Earlier in the day, Nadler conceded that the term "constitutional crisis" is "overused," but said we are in one because "the president is disobeying the law" and "refusing all information to Congress."

The phrase "constitutional crisis" has come up a number of times since Trump entered the White House, perhaps most notably in relation to his travel ban and the firing of former FBI Director James Comey.

In the divisive, chaotic political climate currently pervading the US, it can be difficult to know what it really means when people suggest there's a "constitutional crisis." And the truth is there's no single or simple definition.

Read more: The White House is blocking Don McGahn from cooperating with Congress about the Mueller probe

Generally speaking, a constitutional crisis occurs when one or more of the three branches of government challenges the others for supremacy and throws the US political system out of whack.

As noted by the political scientists Julie Azari and Seth Masket for FiveThirtyEight, there are four types of constitutional crises, and this is a good place to start when trying to grasp what people mean when they utter the phrase.

• When the Constitution doesn't tell us what to do in a given situation.
• When the meaning of the Constitution is unclear and comes into question.
• When the Constitution provides guidelines on what to do, but following through would be impractical or politically unfeasible.
• When the government's institutions and the system of checks-and-balances provided by the Constitution fail.

Political scientist Keith Whittington also provided a useful summary of what might constitute a constitutional crisis in an examination of the subject not long after the impeachment of former President Bill Clinton.

"A political crisis becomes a constitutional crisis when not just a particular administration is put at risk, but the constitutional system itself is tested," Whittington wrote. "To the extent that constitutional crisis is used to mean more than a particularly emphatic sense of political trouble, the term seems to be used to signal the threat of a breakdown in the constitutional order."

"Constitutional crises arise out of the failure, or strong risk of failure, of a constitution to perform its central functions," Whittington added.

He identified two types of constitutional crises: "operational crises and crises of fidelity."

Operational crises occur "when important political disputes cannot be resolved within the existing constitutional framework," Whittington said, and crises of fidelity arise "when important political actors threaten to become no longer willing to abide by existing constitutional arrangements or systematically contradict constitutional proscriptions."

In many ways, whether a constitutional crisis is occurring is in the eye of the beholder. But two of the most prominent examples scholars tend to point to are the Civil War and the Great Depression.

Read more: Over 370 former federal prosecutors say they would have charged Trump with obstruction if he wasn't president

The Civil War was, at its core, a fight over slavery, but also sparked numerous constitutional questions and drastically changed the US Constitution. The Great Depression also catalyzed a multiplicity of questions about the Constitution, the scope of executive authority during times of economic strife (and more generally), and the nature of the Supreme Court.

So, while there's no question that the legislative and executive branches are exhibiting historic levels of friction between one another at present, the question of whether the US is truly facing a constitutional crisis is far more complicated.