I believe this is true, But I'm not 100% sure!
Southern Democrats favored westward expansion, partly to expand cotton plantations and slavery into the west, however, the Whigs were worried about expanding slavery into the west. Most Democrats were in favor of expanding, because it would make America larger. The Whigs, however, were worried about losing representation in the Congress.
Explanation:
The 1848 United States presidential election was the 16th quadrennial presidential election, held on Tuesday, November 7, 1848. In the aftermath of the Mexican–American War, General Zachary Taylor of the Whig Party defeated Senator Lewis Cass of the Democratic Party
Southerners still held many of the resentments that had caused the Civil War