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At the National Constitution Center, in Philadelphia, they like to joke that what’s bad for the country is often good for the organization.

“Web traffic is through the roof,” says the nonprofit’s CEO and president, Jeffrey Rosen. “We had more than 400,000 visitors to our site in the days following Jan. 6,” when supporters of President Donald Trump rampaged in the U.S. Capitol. “Our previous record was around 160,000.”

From his extraordinary political rise in 2015-16 through the four years of his presidency, including his unprecedented challenges to his re-election loss to Joe Biden, Trump’s tenure became a kind of ongoing seminar about how the government works and how a democracy might fail. Questions once limited to Constitutional scholars — how many Cabinet members are needed to invoke the 25th Amendment and remove the president from power, whether a state legislature has the power to overturn the votes of presidential balloting — became part of everyday conversation.

Along with dystopian novels and White House tell-alls, the U.S. Constitution has been a best-seller during the Trump years.

According to NPD BookScan, which tracks around 85 percent of the print market, more than 1 million copies of the Constitution in various editions have sold since Trump took office, compared to around 600,000 during the second term of President Barack Obama. The spike began in 2016 when Trump became the Republican candidate for president: Sales more than doubled from the year before, from 114,000 copies to 275,000, and were nearly four times higher than in Obama’s first year in office, 2009.

A chart shared by BookScan with The Associated Press shows several moments in the Trump presidency that coincided with increased sales of the Constitution: when he formally accepted the Republican nomination, in July 2016; his inauguration in January 2017; the hearings for Supreme Court nominee Brett Kavanaugh in September 2018; and the formal launch by the House of Representatives, in Sept. 2019, of an impeachment inquiry into Trump’s alleged efforts to pressure Ukraine’s president into investigating Biden.

The sales are especially notable because the Constitution can be read or downloaded for free, including from the US government (https://www.archives.gov/founding-docs/constitution-transcript). A pocket edition of the Constitution, published by an imprint of Skyhorse Publishing, has been in the top 20 on Amazon.com for days since Jan. 6, and on Wednesday was listed as out of stock until early next week.

Sanford V. Levinson, a professor at the University of Texas Law School, found it likely the public was responding in part to the president’s own seeming lack of familiarity with the Constitution, citing Trump’s campaign promise to protect a nonexistent “Article XII.”

UCLA law professor Eugene Volokh, who says he has free copies on hand, expressed surprise that so many people would choose to buy the Constitution instead. He wondered if one factor was the kinds of Constitutional questions that Trump helped raise.

“You have people wondering what the emoluments clause says or the impeachment clause,” Volokh says. “I’m sure we’ll have debates under Biden about the the protection clause or the First Amendment, but maybe people won’t buy a copy of the Constitution because they think they know what it says” on those issues.