"He put it that way because the importance of March 15 is, of course, that it is the day in 1820 that Maine, the Pine Tree State, joined the Union.
"Maine statehood had national repercussions. The inhabitants of this northern part of Massachusetts had asked for statehood in 1819, but their petition was stopped dead by southerners who refused to permit a “free state”—one that did not permit slavery—to enter the Union without a corresponding “slave state.” The explosive growth of the northern states had already given free states control of the House of Representatives, but the South held its own in the Senate, where each state got two votes. The admission of Maine would give the North the advantage, and southerners insisted that Maine’s admission be balanced with the admission of a southern slave state lest those opposed to slavery use their power in the federal government to restrict enslavement in the South."