In the prosecution of its purposes, the Slave Power has obtained the control of both the great political parties. Their recent nominations were made to serve its interests, to secure its supremacy, and especially to promote the extension of Slavery.

…

I hear the old political saw, that “we must take the least of two evils.” My friend from Ohio [Mr. Giddings] has already riddled this excuse, so that I might well leave it untouched; but I cannot forbear a brief observation. It is admitted, then, that Cass and Taylor both are evils. For myself, if two evils are presented to me, I will take neither. There are occasions of political difference, I admit, when it may become expedient to vote for a candidate who does not completely represent our sentiments. There are matters legitimately within the range of expediency and compromise. The Tariff and the Currency are of this character. If a candidate differs from me on these more or less, I may yet vote for him. But the question before the country is of another character. This will not admit of compromise. It is not within the domain of expediency. To be wrong on this is to be wholly wrong. It is not merely expedient for us to defend Freedom, when assailed, but our duty so to do, unreservedly, and careless of consequences.

-Abolitionist Charles Sumner, encouraging a third party vote in the 1848 election