After his bold action in the Legislature Lincoln was recognized as the antislavery leader in the central part of Illinois, but was frequently the object of criticism because of his conservative views. He argued, then, as he did twenty-five years later, that the Constitution of the United States was sacred, and as long as it existed must be obeyed. It recognized the right to hold slaves in certain States, and therefore that right could not be denied until the Constitution was appropriately amended. The friends of freedom were at liberty to denounce the great wrong, but they must proceed legally in securing its removal. This position was taken by Lincoln when he was only twenty-eight years old, and he held it until the abolition of slavery became a military necessity. At the same time he was patiently and confidently trying to educate public sentiment and lead the abolition movement in the right direction. Lincoln's second opportunity to place himself formally on record occurred when he was a member of the House of Representatives, where the controversy had been carried long before, and had been revived and vitalized by the treaty with Mexico at the close of the war of 1848, which added to the United States a territory as large as half of Europe. The slave-holders immediately demanded it for their own, but in the previous Congress the Whig and antislavery Democrats had succeeded in attaching to an appropriation bill an amendment known as the Wilmot Proviso, which prohibited the extension of slavery into the territory recently acquired. This had been followed up by the adoption of similar provisions wherever the Whigs could get an opportunity to attach them to other legislation. Lincoln used to say that during his two years in Congress he voted for the Wilmot Proviso in one form or another more than fifty times. Upon his arrival in Washington his horror of the slavery system and the impressions received during his voyages to New Orleans were revived by witnessing the proceedings and the distress in the slave-markets of the national capital, and he determined to devote his best efforts to a removal of that scandal and reproach. Fifteen years later, in one of his speeches during the debate with Douglas, he described the slave-shambles of Washington, and said, "In view from the windows of the Capitol a sort of negro livery stable where droves of negroes were collected, temporarily kept, and finally taken to Southern markets, precisely like droves of horses, has been openly maintained for more than fifty years."