In 1846, Sumner, in a public letter, rebuked Robert C. Winthrop, then a Massachusetts representative, for voting for the Mexican war. In this letter he characterizes the Mexican war as an unjust attack on a sister republic, having its origin in a system of measures to extend slavery; as being dishonorable and cowardly, as being the attack of a rich and powerful country on a weak and defenceless neighbor; and having thus characterized it, he adds: "Such, sir, is the act of Congress to which, by your affirmative vote, the people of Boston have been made parties. Through you they have been made to declare an unjust and cowardly war, with falsehood, in the cause of slavery. Through you they have been made partakers in the blockade of Vera Cruz, in the seizure of California, in the capture of Santa Fe, and in the bloodshed of Monterey. It were idle to suppose that the poor soldier or officer alone is stained by this guilt--it reaches back and incarnadines the halls of Congress; nay, more, through you it reddens the hands of your constituents in Boston. * * * * * "Let me ask you, sir, to remember in your public course the rules of right which you obey in your private capacity. The principles of morals are the same for nations as for individuals. Pardon me if I suggest that you do not appear to have acted invariably in accordance with this truth. * * * * * "It has been said in apology by your defenders that the majority of the Whig party joined with you. * * * In the question of right and wrong it can be of little importance that a few fallible men, constituting what is called a majority, were all of one mind. But these majorities do not make us withhold our condemnation from the partakers in those acts. Aloft on the throne of God, and not below in the footsteps of the trampling multitude of men, are to be found the sacred rules of right which no majorities can displace or overturn. And the question returns, WAS IT RIGHT _to vote for an unjust and cowardly war, with falsehood, for slavery_?" These extracts will give a tolerable certainty that the old Whig party of Massachusetts, which was thoroughly dead in the trespasses and sins of pro-slavery compromise, found Charles Sumner, with all his learning, and vigor and talent, a rather uncomfortable member, and that he soon found that the Whig party was no place for him. In 1848 he left them to unite in forming the Free Soil party, in which the platform of principles he had already announced, was to form the distinctive basis.