Why was Abraham Lincoln compared to Benedict Arnold?

Elected as a Whig to Congress in 1846, Abraham Lincoln gained notoriety when he lashed out against the Mexican War, calling it immoral, proslavery, and a threat to the nation's republican values. President James K. Polk had called for war, accusing Mexico of shedding of "American blood on American soil." Lincoln responded by introducing a series of resolutions demanding to know the "particular spot of soil on which the blood of our citizens was so shed." One of Lincoln's constituents branded him "the Benedict Arnold of our district," and he was denied renomination by his own party.