Blair helped create the Republican Party and supported the first nominee of the party, Abraham Lincoln. Although he had slaves, Blair was said that after the Mexican War that slavery should not be extended beyond where it was allowed. In 1848 he actively supported Martin Van Buren, the Free Soil Party candidate for the presidency. Later, he supported Franklin Pierce, but then went on to help organize the new Republican Party in supporting Anti-Slavery reforms at a convention at Pittsburgh, Pennsylvania in February 1856.