Their 1845 Constitution calls for it...
http://www.snopes.com/history/american/texas.aspThe admission of Texas to the Union posed the potential for upsetting the delicate political balance between free states and slave states. Not only would the annexation of Texas add another slave state to the U.S., but that state would be a vast chunk of territory — nearly four times as large as the then-largest state, Missouri — which would extend the slaveholding portion of the U.S. far beyond its current western boundary. Moreover, the northern portion of Texas intruded beyond the 36°30'N line of latitude which had been established as the demarcation point between free territory and slave territory by the 1820 Missouri Compromise (although the provisions of that compromise technically applied only to "all that territory ceded by France to the United States, under the name of Louisiana" and therefore did not encompass Texas). For their part, some southerners wanted to be able to carve additional slave states out of the huge Texas territory in order to counter the admission of free states and thereby maintain the balance of power between free and slave states in the Senate. The slavery issue (at the time, 90% of Texans were neither slaves nor slaveholders) was addressed in the Joint Resolution for Annexing Texas to the United States, approved by Congress on 1 March 1845, which included a provision allowing Texas to be sub-divided into up to four more states with slavery being banned in states carved out of Texas territory north of the Missouri Compromise line and left up to popular sovereignty in states formed south of the line:
Let's let the folks there vote on this. Who gives a rat's ass what the bugman has to say...he has no power or political office. Where's Ronnie Earle thsse days? Doesn't he still have an indictment with DeLay's name on it?