The conventional wisdom surrounding what happened at the Hartford Convention consequently came to be that failure to support a war effort is likely to mean the death of a political party. Thus, in 1846–1848, when the Whig Party found itself opposed to the Mexican War, the party pragmatists argued that although they might challenge the policy of going to war, they must not fail to vote for funds and supplies to support the army that was fighting it. A rival faction of Conscience Whigs nevertheless took the logical and principled position that if the war was wrong, supporting the fighting of it was also wrong, and that therefore opposition must be thoroughgoing at whatever risk to party fortunes. The resulting divisions within the Whig Party very nearly produced the fatal effect that the pragmatists hoped to avoid.
During the Mexican War the sources of Whig dissent were partially the same as those of Federalist dissent during the War of 1812. In both instances, New England was the stronghold of dissent, although opposition spread more widely from 1846 to 1848. In both instances, the grievances of New England against the administration in Washington included the administration's policies of westward territorial expansion, which implied a permanent diminution of New England's political power. In the Mexican War, of course, westward territorial expansion was immediately at issue. In both instances, fear of the diminution of New England's power sprang not only from direct political interests but from distaste for the whole southern and western economic and cultural system that the dissidents saw represented in the administration in Washington and in the administration's war. During the Mexican War such distaste for southern and western values was reinforced by the rise to prominence of the slavery issue, which had been merely a cloud on the horizon—although already perceptible—during the War of 1812. In August 1846 the Wilmot Proviso tied the slavery issue inextricably to the issues of the Mexican War by proposing to forbid slavery in any territory to be acquired from Mexico. In both instances, opposition to the war could draw upon and be reinforced by the self-conscious Christianity of New England tradition. New England dissent from the War of 1812 had in fact led to the founding of peace societies, which later helped mobilize opposition to the Mexican War.
In the 1840s the Christian antiwar tradition was readily mobilized against a conflict even more iniquitous than the War of 1812, in that the Mexican War could well be regarded—and is still regarded by some historians—as an act of aggression by the strong United States against weak Mexico. The mobilization of this Christian antiwar tradition in its New England centers, at a time when New England happened also to be experiencing its first great literary renaissance, gave an unprecedented literary aspect to dissent against the Mexican War, as illustrated by James Russell Lowell's The Bigelow Papers (1848) and Henry David Thoreau's essay "Civil Disobedience" (1849).
For all that, organized dissent against the Mexican War never attained a climax as notable, albeit ambiguous, as the Hartford Convention. The military campaigns of the war proved to be short and unvaryingly successful, which in turn proved an insurmountable handicap to effective dissent. Military success in fact diverted the pragmatic, political Whigs from the issue of whether to vote supplies to the more expedient issue of how to capitalize on the military fame of the victorious generals, Zachary Taylor and Winfield Scott, who chanced also to be Whigs.
Nevertheless, opposition to the Mexican War tended to grow more intense the longer the war lasted. When the conflict began in May 1846, only two members of the Senate and fourteen members of the House of Representatives voted against the bill, declaring that war existed "by the act of the Republic of Mexico." Congress authorized the president to call volunteers and appropriated $10 million for the conduct of the war. By the time the Thirtieth Congress assembled for its first session in December 1847, however, to be greeted by President James K. Polk's message that no peace had yet been obtained and there was no immediate prospect of one, Congress appeared much less ready to vote for more men and money, and certainly the Whigs were more determined to pin upon Polk and the Democrats responsibility for a war begun, as they interpreted it, not by Mexico but by American aggression. Furthermore, the Whigs had captured at least nominal control of the House, although their own divisions made their election of Robert C. Winthrop of Massachusetts as Speaker a very near thing, because the most dedicated Conscience Whigs refused to support Winthrop as too willing to sustain the war. The House then defeated a resolution declaring the war just and necessary. Opposition to slavery inevitably still influenced much of the opposition to the war, but even a southern Whig, Alexander H. Stephens of Georgia, could say, "The principle of waging war against a neighboring people to compel them to sell their country is not only dishonorable, but disgraceful and infamous." Playing upon the fact that hostilities had begun when Mexican troops attacked Taylor's forces after they had crossed south of the Nueces River, which Mexico claimed as the southern boundary of Texas, Representative Abraham Lincoln of Illinois introduced his "spot" resolutions calling on the president to say candidly whether the spot where the war began was Mexican or American soil.
Some powerful Democrats, too, had grown outspokenly critical—the elderly Albert Gallatin, who called it a war of subjugation; Thomas Hart Benton; even John C. Calhoun, who feared the war was becoming one for the conquest of central Mexico, which would bring into the Union a racially inferior people incapable of free government. Fortunately for the president, in the midst of congressional debate there arrived the Treaty of Guadalupe Hidalgo (1848), negotiated in Mexico by Nicholas P. Trist, which ended the war with the annexation of Texas confirmed and California and New Mexico added to the United States, in exchange for a payment of $15 million and the assumption by the United States of American claims against Mexico. Although Polk had earlier repudiated Trist as his negotiator, and although some Democrats thought Trist's terms too generous toward Mexico, Polk submitted the treaty for ratification lest the increasingly noisy dissent prove able to paralyze him and make a good treaty henceforth impossible. Under public pressure to end the war as swiftly as could be done, the Senate ratified the treaty.
The opposition Whigs became the immediate political beneficiaries of discontent with the war. Some Conscience Whigs split off from the party to join with various northern Democratic factions disgruntled over the pro-Southern tendencies of all Polk's policies and to form the Free-Soil Party for the election of 1848; but this third party hurt the Democrats more than the Whigs. Choosing Zachary Taylor as their presidential nominee, the Whigs carried the White House. The expedient course of the party pragmatists in supporting war measures if not the war itself apparently had accomplished far more than merely warding off the fate of the Federalists. But the Whig success of 1848 was deceptive. Wartime strains upon the relations between Conscience Whigs and pragmatists had so weakened the party that it could not survive another bout with the slavery issue. The slavery crisis of 1850 broke open the cracks imposed on the party structure by the war and destroyed the party.