Why did voters leave the Whig and Democratic Parties to join the Free-Soil Party in 1848?

Voters left the Whig and Democratic Parties to join the Free-Soil Party in 1848 primarily due to growing concerns over the expansion of slavery into new territories acquired from Mexico. Many Northerners, particularly abolitionists and those who opposed the political power of slaveholders, were dissatisfied with their parties' stances on slavery. The Free-Soil Party, advocating for "free soil, free speech, free labor, and free men," attracted those who wanted to prevent the spread of slavery and promote a vision of a nation where free labor would thrive. This shift reflected the increasing polarization of American politics around the issue of slavery.