Southern voters represented the biggest potential impediment in Van Buren's quest for the presidency, as many were suspicious of a Northern president. Van Buren moved to obtain the support of southerners by assuring them that he opposed abolitionism and supported the maintaining of slavery in states where it had already existed. To demonstrate consistency regarding his opinions on slavery, Van Buren cast the tie-breaking Senate vote for a bill to subject abolitionist mail to state laws, thus ensuring that its circulation would be prohibited in the South. Van Buren considered slavery to be immoral but sanctioned by the Constitution.