Who formed the Free-Soil Party and why?
Free Soilers were formed by New York and Illinois anti-slavery elements of the Democratic and Whig parties.
No one person leaps out as the founder of the Free Soil party (although Illinois politician and educator Willard Woodard is credited as a co-founder). They nominated two candidates to the presidency, Martin Van Buren in 1848 and John P. Hale in 1852. The poet Walt Whitman was a member, as was Treasury Secretary Salman P. Chase. The party was very influential in New York, Illinois and Massachusetts, but was of limited appeal elsewhere in the Union.
The Free Soilers were not abolitionists; Van Buren, who opposed slavery on moral grounds but acknowledged that it was endorsed by the Constitution, typified the party's attitude towards slavery. They specifically opposed allowing it in the territories newly gotten from the Mexican War, but did not seek to ban it in the existing slave states of the south.
Their electoral antics are believed to have "spoiled" the 1848 election in Whig party nominee Zachary Taylor's favor and their 1852 run was even less effectual. In 1854, the Free Soil and Whig parties disbanded and together formed the Republican party, which had anti-slavery language in its platform. With the election of Abraham Lincoln in 1860, they have remained one of America's two most significant political parties ever since.