If James Marshall started the Gold Rush, it could be said James K. Polk got it into high gear.
Polk became president in 1844. He won the Democratic nomination only after delegates could not agree on any other candidate, and then only after he agreed not to seek a second term. Largely a political unknown outside his native Tennessee, Polk narrowly defeated Henry Clay in the general election.
As chief executive, Polk had a clear agenda for expanding the country. He particularly coveted California, even though he never set foot in it, and was willing to buy it from Mexico. But Mexico didn't want to sell. So Polk provocatively put a small American army on the border, a Mexican force attacked it, and the Mexican War was on.
In the aftermath of the war and the ceding of vast areas by Mexico to the United States, Polk found himself scathingly criticized for its high cost. So when a report came from California in late 1848 that gold had been found, Polk was quick to seize on it as further, if after-the-fact, justification of the war.
"The accounts of the abundance of gold in that territory are of such an extraordinary character as would scarcely command belief, were they not corroborated by the authentic reports of officers in the public service," Polk said in a message to Congress in December 1848.
Prior to Polk's address, many people believed the discovery of gold was just another Western fable. In those days, however, people still believed presidents. Polk's words made it official: The Gold Rush was on in earnest.
But Polk did not live to see it at its height. The president who launched a hundred thousand prospectors died of cholera in June 1849, a few months after leaving office.