Polk served as the Speaker of the House of Representatives and Governor of Tennessee before he entered the competition for Presidential Candidate.
He had a long and distinguished career, but was actually thought to be a Vice Presidential Candidate of the Democratic party, not a presidential candidate. The Democrats were expected to choose Martin Van Buren who had actually served as president two administrations ago.
What made Polk into a serious contender on the national stage and then the President of the United States were his views on manifest destiny.
During the Whig and Democratic Convention of 1844, Henry Clay, the Whig candidate and Van Buren, the expected Democratic candidate had both come out against the annexation of Texas, badly misreading public opinion at the time to be against the expansion of the country. Polk however came out strongly in favor of annexing Texas.
Clay and Van Buren both thought that the leading issue of the day, slavery and its potential to expand would cause most Northerners to be against the annexation of Texas, but they were wrong.
Polk was the new hero of the people and quickly went from vice presidential candidate to President Elect, adding another sad chapter to the ballad of Martin Van Buren and Henry Clay.