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It is actually conceivable that the Wilmot Proviso might have passed the Senate in 1846. At that time it was almost purely a sectional, not a party issue--northern Democrats voted almost unanimously for it in the House. If they were to back it unanimously in the Senate, just one vote from a border slave state--John Clayton of Delaware is the most plausible one, though even he is doubtful--the Two Million Dollar bill (appropriations for making peace with Mexico) might pass with the Proviso in it.

The reason the Proviso did not come up for a vote in the Senate in 1846 is a little weird. "The Senate took up the bill late in its Monday session. Southern Democrats hoped to reject the Wilmot Proviso and send the bill back to the House for a quick approval of the bill without the restrictions on slavery. Whig John Davis of Massachusetts attempted to forestall this effort by holding the floor until it would be too late to return the bill to the House, forcing the Senate to accept or reject the appropriation with the proviso intact. However, before he could call the vote, due to an eight-minute difference in the official House and Senate clocks, the House had adjourned and the Congress was officially out of session..." https://en.wikipedia.org/wiki/Wilmot_Proviso Some people have argued that Davis was actually secretly opposed to the Proviso and wanted to kill it; others that he was simply too loquacious and didn't realize what he was doing until it was too late. But all the evidence seems to indicate that he did indeed favor the bill with the Proviso attached.

David Potter writes "A different result would have changed the course of American history, and there has been much speculation as to whether the measure could have passed, with the Proviso in it, if it had come to a vote." https://books.google.com/books?id=S7Qk9nIwk14C&pg=PA23 Wilmot himself, Salmon Chase, and Jacob Brinkerhoff all thought it would have; Henry Wilson in History of the Rise and Fall of the Slave Power in America, Volume 2 argued they were wrong: "It was taken up in the Senate on the last day of the session, which was to close at noon, and a motion was made to strike out the proviso. John Davis of Massachusetts took the floor, and, he declining to yield it, the bill and proviso were lost. Mr. Davis was much censured at the time for not permitting a vote to be taken. But, whatever were his motives, it is probable that a vote could not have been reached on the motion to strike out the proviso; and, if it had been, it would have unquestionably prevailed, as there was a majority of slaveholders in that body, and the exigencies of the system would not have allowed them to see the purpose of the war thus defeated. It has indeed been since affirmed by Mr. Brinkerhoff that there was 'a well-ascertained and unanimous determination on the part of the Democratic senators of the free States to stand by the proviso, and that those of Delaware and Maryland would have voted with them.' But surely Mr. Brinkerhoff must have been mistaken. It is barely possible that Democratic senators from the free States would have voted for that measure, but their previous and subsequent conduct does not justify the belief that they would have done so. Mr. Pearce of Maryland and the two Delaware senators are not living to speak for themselves, but the subsequent course of Mr. Pearce and John M. Clayton gave no assurance that they would have voted for the proviso had it come to a vote. The probability is strong that they would have voted against it, and Reverdy Johnson, in a letter written in April, 1873, states in the most unequivocal language that he should not have voted for it." https://books.google.com/books?id=3Y0-AAAAYAAJ&pg=PA17 (OTOH, Michael Holt notes in The Rise and Fall of the American Whig Party that "Prior to his inclusion in Taylor's cabinet, Clayton had been the only southern Whig senator ever to cast a vote for the Wilmot Proviso..." https://books.google.com/books?id=5aGyVFn3VnMC&pg=PA565 He also notes that Clayton had been a staunch supporter of the Taylor plan to admit California and New Mexico as free states. Maybe Henry Wilson was a little too quick to assume that anyone he disagreed with politically was "pro-slavery.")

So suppose the bill does pass. The obvious and probably correct reply is that it makes no difference, because Polk promptly vetoes it. Polk had reacted with irritation to the original introduction of the Proviso: "What connection slavery had with making peace with Mexico it is difficult to conceive." But Polk did not expect slavery to take root in the Southwest, and the South was not yet as up in arms against the Proviso as it would soon be. (See James L. Huston, Calculating the Value of the Union: Slavery, Property Rights, and the Economic Origins of the Civil War, p. 155 https://books.google.com/books?id=DqnqCQAAQBAJ&pg=PA155 on how slow the South was to react to the Proviso.) What if Polk decides that for the sake of peace with Mexico, the Two Million Dollar bill is worth signing, "mischievous & foolish" https://books.google.com/books?id=C8ufqRJuwy8C&pg=PA228 Proviso and all?

Within a couple of years, there would be no chance of the Proviso passing the Senate, as many northern Democrats came around to "squatter sovereignty" as an alternative. But just maybe in 1846 there was a window of opportunity for the Proviso. But is Potter correct in saying that its passage (even assuming no veto) would have changed the course of US history? Unless repealed--or found unconstitutional by the Supreme Court--it would mean no African American slavery in the Southwest but in practice there was practically none anyway; California was of course admitted as a free state, and ten years after the Compromise of 1850 theoretically opened the New Mexico and Utah Territories to slavery, the 1860 census showed only twenty-nine slaves in Utah and none at all in New Mexico--even though the New Mexico legislature had enacted a slave code! (Don Fehrenbacher quipped that New Mexico "perversely became the only jurisdiction in American history to enact a slave code for a slaveless society." https://books.google.com/books?id=3KYFlabs2hQC&pg=PA84 As another scholar observed, this is not really true because it ignores "thousands of Indian captives and Hispanic debt peons detained in systems of coerced servitude..." https://books.google.com/books?id=kDaXDgAAQBAJ&pg=PA116 But it is doubtful that the Proviso would have changed their condition.) Of course it is possible that one of the southern soldiers stationed in New Mexico would have brought a suit to establish his right to bring his slaves with him, and we would thus have had a Dred Scott decision some years earlier than in OTL.

Assuming no earlier Dred Scott-like case, what happens when the time comes to organize Kansas and Nebraska as territories? Arguably, being shut out of the Southwest by the Proviso will make southerners all the more determined to repeal the Missouri Compromise to allow slavery in Kansas. But will they get sufficient northern support? In OTL Douglas and other northern supporters of repeal could point to the Compromise of 1850 as establishing the "principle of popular sovereignty." Cass in 1848 had already prepared the Democrats for this doctrine. Of course the fact that the principle was applied to the Mexican Cession hardly means that it was intended to apply to all US territory and supersede the Missouri Compromise but it is not necessary to dwell on the sophistical nature of Douglas' reasoning. The point is that there was at least superficially a precedent for squatter/popular sovereignty. Without that precedent, would the application of the principle to Kansas and Nebraska still get the necessary majority?

Of course one might say that Dred Scott would have invalidated the Missouri Compromise (and the Wilmot Proviso) anyway. But it was by no means inevitable that the Supreme Court would decide Dred Scott on such broad grounds; its decision to do so was in part a result of the peculiar political situation of 1856-7, which might be different in this ATL.
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