Popular support for the war effort initially appeared strong (by some estimates, more than 200,000 men responded to the War Department's call for 50,000 volunteers in May, 1846). But the surge of patriotic feeling was short-lived. Whig leaders were disturbed by the sweeping new opportunities for executive power and patronage which the war had created for the Democrats. More ominously, many northern politicians of both parties were troubled by the fact that the expansionist president appeared determined to acquire vast new territories for the benefit of the slaveholding South. The aggressiveness with which the administration had worked to obtain Texas, and its apparent determination to acquire additional territory in the Southwest at Mexico's expense, had given rise to feelings of betrayal among northern Democrats, reinforcing their suspicions that the president's priorities were southern ones.
As a result, opposition to the war grew steadily, even as U.S. armies racked up a string of victories in Mexico. By the fall of 1847, the Whigs had won a majority in the House of Representatives. In New England, "Conscience Whigs" condemned the conflict as an immoral conspiracy to extend the slave empire, a sentiment echoed with increasing conviction by Northern Democrats, who were rallying behind the Wilmot Proviso in an effort to prohibit slavery in the lands seized from Mexico. Fearful of the war's impact on party unity, statesmen of both parties called upon the administration to formally disavow any interest in acquiring Mexican territory.
Critics of the administration included Abraham Lincoln, a freshman Whig representative from Illinois. In late 1847 Lincoln presented to Congress a set of resolutions condemning the war as unnecessary and unconstitutional. The resolutions became known as the "spot" resolutions, because Lincoln demanded to know the exact spot where American blood had been spilled.