Though he lacked the broad support of Republican Senator William H. Seward of New York, Lincoln believed that he could emerge as the Republican presidential nominee at the convention after multiple ballots. Lincoln spent much of 1859 and 1860 building support for his candidacy, and his Cooper Union speech was well-received by eastern elites. Lincoln positioned himself in the 'moderate center' of his party; he opposed the expansion of slavery into the territories but did not favor the abolition of slavery in slave states. Shortly after McClellan's return to command, General Lee's forces crossed the Potomac River into Maryland, leading to the Battle of Antietam in September 1862. The ensuing Union victory was among the bloodiest in American history, but it enabled Lincoln to announce that he would issue an Emancipation Proclamation in January.