Harrison Gray Otis was a U.S. senator from Massachusetts. Suddenly awakening to the political value of an anti-slavery crusade, he quickly joined the conspiracy and, in two letters, issued instructions to William Sullivan, his fellow Federalist in Boston, to implement the conspiracy there.

The two letters are an excellent description of the purpose of the anti-slavery conspiracy and its mode of operation. The letters fortunately survived instructions to burn after reading.

Otis had not immediately recognized the purpose of the sudden anti-slavery agitation in Congress. Neither did his fellow Federalist senator Rufus King of New York when Otis told him of the agitation in the House.

Both Otis and King, more aligned with the commercial interests in Boston and New York, had voted against the increase in the duty on iron. Although both men resented the political power of the cotton South, they were slow to recognize the political significance of anti-slavery. Once awakened to the prospect of depriving the South of the balance of political power, both men eagerly joined the anti-slavery crusade. King became the most vociferous anti-slavery zealot in the Senate.

Otis famously voted against the tariff of 1820, sending it down to defeat by a one-vote margin.

Otis soon became an investor in the Taunton Manufacturing Company and changed his attitude on the protective tariff. He apologized in public for his previous vote against the tariff.

Otis joined Lewis Tappan's movement for the tariff and gave an important speech at Tappan's Boston tariff convention in 1827.