New York - Sales of a pocket version of the US Constitution have soared since the father of a fallen Muslim American soldier brandished a copy to denounce Donald Trump, igniting one of the biggest backlashes against the Republican nominee.Khizr Khan, whose son died in a 2004 suicide bombing in Iraq, has gone head to head with the insult-dishing White House candidate in a confrontation that has dominated the US news cycle for days.On Monday, a pocket edition of the constitution was second only to the new "Harry Potter and the Cursed Child" script on Amazon's best seller list, and number one best seller in Amazon's US history category.The 52-page paperback, on sale for $1 on Amazon, does not appear to be the same edition that Khan whipped out of his pocket in one of the most defining moments of last week's Democratic National Convention.The online retailer did not immediately respond to a request for comment.Trump on Monday renewed his assault against Khan as families of other fallen US soldiers demanded the tycoon apologise and prominent Republicans criticised 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."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.