This was a common impression among active abolitionists.
It is not strange, therefore, that they refused to support him as a candidate for the Presidency, and it is not at all certain that his election in 1844 would have prevented the war with Mexico.
Northern Whigs accused the Democrats of fomenting a war with Mexico with the intention of gaining territory for the purpose of extending slavery.
Democrats denied that the annexation of Texas would lead to war, and many of them proclaimed their opposition to the farther extension of slavery.

In harmony with this sentiment, when President Polk asked for a grant of two million dollars to aid in making a treaty with Mexico, they attached to the bill granting the amount a proviso to the effect that slavery should forever be prohibited in any territory which might be obtained from Mexico by the contemplated treaty.

The proviso was written by an Ohio Democrat and was introduced in the House by David A.Wilmot, a Pennsylvania Democrat, after whom it is known.

It passed the House by a fair majority with the support of both Whigs and Democrats.

At the time of the original introduction in August, 1846, the Senate did not vote upon the measure.