For the election of 1844, the Democrats nominated Speaker of the House James K. Polk on a platform supporting the annexation of Texas and demanding all of the Oregon Territory to the 54° 40' line. The Whigs, having formally kicked President John Tyler out of their party, Whigs selected Henry Clay (his third presidential bid). The new antislavery Liberty Party also nominated a weak candidate, mainly for show. In the end, though, Polk won 170 electoral votes to Clay’s 105; surprisingly, the Liberty Party stole just enough votes from Clay to tip the election toward the Democrats.
Tyler, concluding that Polk’s victory was a mandate from the American people to annex Texas, put the issue to a vote in both houses of Congress. Thus, in 1845, Congress officially annexed the Lone Star Republic. Mexico was outraged: they had refused to recognize Texas independence in 1836 and believed that the rebellious state would one day be reconquered. After the annexation announcement, Mexico withdrew its ambassador from Washington, D.C.
Then, a border dispute exacerbated the situation: whereas the United States claimed that Texas extended all the way south to the Río Grande, Mexico claimed that Texas was smaller, ending further north at the Nueces River. Both sides sent troops to the region, the Americans camping north of the Nueces and the Mexicans to the south of the Río Grande.
Polk went to the White House with a specific “to-do” list and accomplished all of his goals by the time he left. First, with the Walker Tariff of 1846, he reduced the tariff that had crept higher and higher since 1842. The new tariff set taxes on foreign goods at around 35 percent. Second, Polk reestablished the independent treasury that Martin Van Buren had created and that Tyler had decommissioned.
In addition, Polk wanted westward expansion, especially into Oregon and California. California had recently become a hot topic and prize in the West for its San Francisco Bay. However, acquisition of California would be difficult: it belonged to Mexico, which was not on good terms with the United States.
Acquiring Oregon was not difficult but did spark controversy. Recognizing that it could never win the population war in Oregon, Britain proposed giving the United States all of Oregon south of the 49th parallel rather than quarreling for the entire territory up to 54° 40'. The Senate agreed to the compromise, despite protests from many Americans who wanted the entire territory.