Sales of the U.S. Constitution surged over the past four years, and interest was recently renewed following the Capitol Building riot.

NPD BookScan, which tracks 85% of the print market, found that Constitution sales began to spike in 2016, when Donald Trump became the candidate for president. Since then, more than 1 million copies have been sold, and were doubled from the year before, when President Barack Obama was in office, according to the Associated Press. Sales were four times higher when Trump was the Republican candidate than in Obama’s first year in office, the AP reported.

The National Constitution Center in Philadelphia also noticed a significant increase in interest in the Constitution in the days following the breach of the Capitol Building, when demonstrators stormed the complex while the Senate was scheduled to certify the Electoral College results.

National Constitution Center’s CEO and president Jeffrey Rosen said“Web traffic is through the roof. We had more than 400,000 visitors to our site in the days following Jan. 6. Our previous record was around 160,000.”

NPD reported that Constitution sales increased 60% since 2017, with an average of almost 20 thousand sales each month, as of October 2019. Sales spikes occurred during sensational moments in the presidency, and when Trump entered the political scene just prior to entering the White House.