Martin was elected as a delegate to the Constitutional Convention of 1787 in Philadelphia. When he arrived on June 9, 1787, he expressed suspicion of the secrecy rule imposed on the proceedings. He also opposed the creation of a government in which the large states would dominate the small ones, he consistently sided with the small states, helping to formulate the New Jersey Plan and voting against the Virginia Plan. On June 27, Martin spoke for more than three hours in opposition to the Virginia Plan's proposal for proportionate representation in both houses of the legislature. He is known for his warm opposition to the development of a strong central government, as well. He was known for his ability to talk and as stated by William Pierce "he was educated for the Bar... and he never speaks without tiring the patience of all who hear him."