In February 1804, Cooper attended a dinner party during which Alexander Hamilton spoke forcefully and eloquently against the Federalists' plan to nominate Aaron Burr as their candidate for Governor of New York. Cooper later wrote a letter to Philip Schuyler, Alexander Hamilton's father-in-law, in which he made reference to the "despicable opinion" Hamilton had expressed about Burr. The letter was published in "The Albany Register", but was tame compared to other attacks on Burr in the press. Still, Cooper's letter proved the last straw in the ongoing rivalry between Burr and Hamilton. When Burr read the letter weeks later, shortly after his defeat in the governor's race, he was enraged by Hamilton's alleged remarks, and challenged Hamilton to a duel, in which Hamilton was killed.