Senator Martin Van Buren, who had been a prominent supporter of Crawford in the 1824 election, emerged as one of the strongest opponents of Adams's policies, and he settled on Jackson as his preferred candidate in the 1828 election. Jackson also won the support of Vice President John C. Calhoun, who opposed much of Adams's agenda on states' rights grounds. Across the South, reaction to the abolition mail campaign bordered on apoplexy. In Congress, Southerners demanded the prevention of delivery of the tracts, and Jackson moved to placate Southerners in the aftermath of the Nullification Crisis. Postmaster General Amos Kendall gave Southern postmasters discretionary powers to discard the tracts, a decision that abolitionists attacked as suppression of free speech.Another conflict over slavery in 1835 ensued when abolitionists sent the U.S. House of Representatives petitions to end the slave trade and slavery in Washington, D.C. These petitions infuriated pro-slavery Southerners, who attempted to prevent acknowledgement or discussion of the petitions. Northern Whigs objected that anti-slavery petitions were constitutional and should not be forbidden.