Martin served as a delegate to the Maryland State Convention of 1788, to vote whether Maryland should ratify the proposed Constitution of the United States. Most of the delegates at the convention ignored Martin's warnings. In April 1788, the majority of the delegates voted to ratify the Constitution, making Maryland the seventh state to do so. In June, when New Hampshire became the ninth state to ratify, the required threshold had been reached, and the new Constitution took effect. Three years later, the first 10 amendments were added. Around 1791, however, Martin turned to the Federalist Party because of his animosity toward Thomas Jefferson, who, in 1807, spoke of him as the "Federal Bull-Dog."