When he took his seat in the Twenty-ninth Congress the issue in regard to the extension of slavery in the Territories was beginning to enlist public interest.
The first impulse of all the representatives from that extensive and opulent domain, which had been saved from the blight of slavery by the Ordinance of 1787, was to aid in extending a similar blessing to all other Territories of the United States.

With the exception of Stephen A.Douglas and John A.McClernand of Illinois, and John Pettit of Indiana, all the Democratic representatives from the four North-western States (Ohio, Indiana, Illinois, and Michigan) voted for the anti-slavery proviso offered by Mr.Wilmot.

Mr.Douglas, discerning the future more clearly than his party associates, realized that the chief strength of the Democracy must continue to lie in the South, and that an anti-slavery attitude on the part of the North-western Democrats would destroy the National prestige of the party and lead to its defeat.

The Democratic supporters of the Wilmot Proviso had therefore choice of two paths: they must abandon their anti-slavery attitude or they must leave the party.

Mr.Thurman adhered to his party.