Answer :

Southerners supported the annexation of Texas as a slave state. Due to its proximity to cotton producing and slave owning states, delegates from those states wished to see Texas brought in as a slave state thereby adding to the slave states' total numbers in the House and Senate. This was particularly difficult, however, because when Texas belonged to Mexico, slavery was prohibited. Following its independence and annexation, however, wealthy landowners in the region finally had the opportunity to bring about slave ownership.
The supporters the annexation Texas as a slave state. The southerners wanted slavery while the northerners didn't want slaves. Texas was in the south.