Former President James K. Polk and his dream to fulfill the Belief of Manifest Destiny had the most significant economic and Political impact on the United States during the westward expansion in the 19th Century, because he got land from Mexico for southwest territory, and he fulfilled all of his champaign promises.
"[James K. Polk] was president when the United States added the greatest amount of new territory to the country. During his presidency, the American flag was raised over most of the area now forming 9 western States, and Texas became a member of the Union."
"A workaholic, America's new chief executive set am ambitious agenda with four major goals:cut terifs, reestablish an independent US treasury, secure Oregon territory and acquire the territories of California and New Mexico from Mexico. Polk achieved his goals."
"[Polk] was a champion of manifest Destiny-the belief that the United States was fated to expand across the North American continent- and by the end of his four years in office, the nation expanded, for the first time, from the Atlantic Ocean to the Pacific Ocean."