Congressional Democrats suffered huge losses in the mid-term elections of 1854, as voters provided support to a wide array of new parties opposed to the Democratic Party. Most victorious congressional candidates who were not affiliated with the Democratic Party had campaigned either independently of the Whig Party or in fusion with another party. "Bleeding Kansas", a struggle between anti-slavery and pro-slavery settlers for control of Kansas Territory, escalated in 1855 and 1856, pushing many moderate Northerners to join the nascent Republican Party. As cooperation between Northern and Southern Whigs appeared to be increasingly impossible, leaders from both sections continued to abandon the party. In September 1855, Seward led his faction of Whigs into the Republican Party, effectively marking the end of the Whig Party as an independent and significant political force. In May 1856, after denouncing the Slave Power in a speech on the Senate floor, Senator Sumner was attacked by Congressman Preston Brooks, outraging Northerners. Meanwhile, the 1856 American National Convention nominated former President Fillmore for president, but many Northerners deserted the American Party after the party platform failed to denounce the Kansas–Nebraska Act.