"Today was the busiest day so its definitely accelerating," said Keir Walton, president of Oak Hill Publishing Company, founded in 1993 in Chicago.
"This is the first thing I've seen in 23 years that's just fueled this incredible acceleration," he told AFP by telephone.
Walton said he had dispatched 150,000 new copies to replenish stocks and was "100 percent certain" that the hike in sales was linked to Khan's appearance at the Democratic convention.
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"It's super exciting that it's reintroduced so many people to the constitution, because it's a fascinating document as relevant today as when it was written," he said.
Trump on Monday renewed his assault against Khan as families of other fallen US soldiers demanded the tycoon apologize and prominent Republicans criticized his disparagement of the bereaved father and mother.
Khan, a Pakistani immigrant, last Thursday stood next to his wife accusing Trump of vilifying patriotic American Muslims, saying that if the Republican had his way his son, Captain Humayun Khan, would never have been in America.
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"You are asking Americans to trust you with their future, let me ask you -- have you even read the United States Constitution?" he said, waving a copy to loud cheers at the Democratic convention in Philadelphia.
"I will gladly lend you my copy!" Khan said, demanding Trump consult the 14th amendment, which guarantees equal protection before the law.
As soon as Khan finished speaking, "register to vote" spiked as a Google search.