Why was Stephen Phillips against the annexation of Texas?

Phillips was mayor of Salem from 1838 to 1842, but was defeated as the Free-Soil candidate for governor in 1848 and 1849. The Free Soil Party was a short-lived political party in the United States active in the 1848 and 1852 presidential elections as well as in some state elections. A single-issue party, its main purpose was to oppose the expansion of slavery into the Western territories, arguing that free men on free soil constituted a morally and economically superior system to slavery. Texas would have been annexed as a slave state.