This thread got me interested and I was doing some research and apparently one of the factors believed to have contributed strongly to Polk's early death was that he probably contracted cholera during a good will tour of the south during the waning days of his first term. Our POD could be for this not to happen thereby giving us counter-factual historians the excuse to extend Polk's life by say another 10 years (he lives until he's 63 in 1859 in TTL)
However even without contracting cholera, I think that given Polk's promises during his campaign in 1844 and his achievements in his first term a second term in 1848 is not going to happen. Polk is still going to suffer from overworking himself during his first term and will gladly retire to private life in 1849. Yet I think, given the troubles the country undergoes with the compromise of 1850 and the deadlocked convention of 1852, Polk's name will quickly come to the forefront and win the nomination handily. In this case James Buchanan could very well end up being Polk's VP.
Now this might affect the Whig convention as it was held in OTL 2 weeks after the Democratic one. With the Democrats nominating the "superstar" candidate James K. Polk, the Whigs are even less inclined to re-nominate Fillmore. There's just no way they're going to win in the South against Polk. However due to their drastically decreased chances of winning, I have my doubts that Winfield Scott is going to be more hesitant than in OTL to enter the race. Personally, I think Scott will instead hedge his bets on 1856 as Polk will be barred by tradition from running for a third term and Scott would in this scenario be facing a potentially weaker Democrat.
That being said, numerous delegations still support Scott's bid for the presidency and 2 weeks prior to the convention is a poor time to drop out. Perhaps a deal is struck with the perennial Whig Candidate Daniel Webster? In exchange for picking Scott as his VP, Scott's delegates will transfer to Webster during the convention. Webster wholeheartedly agrees because he has nothing to lose, and Scott is an excellent VP choice. Scott's supporters on the other hand see this as a chance to win southern votes and a means to get Scott into the white house as Webster is by this point quite ill and most likely won't serve out his entire term.
Come the convention, Webster and Scott's delegates annihilate those supporting Fillmore. Perhaps this leads to the southern Whigs breaking away and nominating Fillmore for a rival presidential bid (Just like some Georgia Whigs nominated Webster in OTL). I could see such a quick and geographically polarized convention doing so in TTL (by showing their dominance through universal support of the Webster/Scott ticket, the Northern Whigs basically tell the Southern Whigs that they have no place in the party, that's a recipe for a split if there ever was one).
TTL's election campaign proceeds with Polk doing much better than Pierce south of the Mason Dixon line and slightly worse than Pierce in the Northern Free states. Let's say in this TL, the Southern Whigs storm out of the convention and re-nominate Fillmore for his own run at the Presidency. With the Whig party split between north and south and with the Democrats running Polk, slavery will undoubtedly feature heavily in the campaign. I'd expect the northern whigs and abolitionists to do slightly better than OTL in the northern states at the expense of Polk. Meanwhile Fillmore could try to ignore slavery altogether and instead focus on anti-immigrant fervor in the North and defense of slavery in the South.
Support for the Whigs in TTL is tempered by their problematic ticket. Northerners view Webster as too pro-slavery and driven by political gain, while Southerners view Scott as far too friendly towards the abolitionists and a likely president given the poor health of Webster much like 2008's perception of the McCain/Palin ticket. This is exacerbated when in October of 1852, Daniel Webster dies due to liver complications (a week or two earlier than OTL due to the increased stress of the campaign). This makes Scott the Northern Whig nominee for POTUS which drastically increases support in the North and drastically decreases it in the South despite several statements made by Scott in opposition to the Free Soil movement.
Come election day, Polk wins the electoral college handily winning every state south of the Mason-Dixon line by a large margin (over 5%). In the north, however, Polk's ardent support for slavery and his willingness to compromise over the Missouri compromise/expand the slave holding parts of the Union (to maintain the country) hurt him in the North. However the split between the Northern Whigs, the Free Soilers/Liberty Party, and the Know-Nothings lead to him winning almost every state there as well. That being said, every victory Polk wins in the North is by a plurality of under 50%. The popular vote is much more telling with the Democrats losing almost half of their seats in the North to Whig/Free Soil/Know Nothing candidates.
This results in the 33rd Congress having a much closer house of Representatives.
33rd Congress House of Representatives
Democrats:118
Whigs: 87
Know Nothings:19
Free Soil: 10
(In this scenario, All southern Whigs became Know-Nothings, and the Democrats lost half their congressional support in the North with it being parceled out roughly evenly between the Whigs, the Know Nothings, and the Free Soilers picking up the scraps. I think it's pretty realistic.)
A quick glance at the popular vote reveals that had the Northern Whigs and the Free Soilers combined their respective votes, they would have (narrowly) carried enough Northern states to win the election regardless of Polk's supremacy in the South and California. This sets said parties to begin talks for the coming electoral year, along with promises of cooperation in Congress.
Now where this gets interesting is as one poster above noted, the Kansas-Nebraska act. As the Senate in TTL remains more or less unchanged (strong Democratic majority) the first part of the bill goes much like OTL. The initial Nebraska act is still passed by the House of Representatives and fervently debated and revised in the Senate to favor the slave holding states. However when it comes back to the House of Reps, the bill hits a stone wall as in TTL the democratic majority is not nearly so strong. Though attempts are made to get Know-Nothing Support for the bill, the debate has made it a toxic issue in the North. The result is that Southern Know Nothings vote for it and Northern ones against it, furthermore the bill is staunchly opposed by Northern Whigs and Free Soil representatives as well as the vast majority of Northern Democrats who in TTL were devastated by Polk's support for slavery in 1852 (there's only half of them left). The result is that the bill fails on the floor of the House of Representatives. An attempt is made to revise the bill but the differences between the two sides are insurmountable and the effort fails.
The failure of the Kansas-Nebraska act does two things. 1) It solidifies the anti-Democrat coalition in the North, especially between Northern Whigs, Free Soilers, and Northern Know-Nothings. Conversely it also strengthens southern support for the Democratic party. 2) It pressures Polk to look for expansionist avenues of alleviating the situation in light of the failure of the Kansas-Nebraska act. Cuba, the Danish Virgin Islands, and William Walker's expedition into the Baja area are all examined as possible outcomes. The first two probably garner a great deal of opposition from the North but the last one, if finessed properly, could lead to a second US Mexican War.
Let's say in TTL, Polk/Southern Democrats see to it that Walker's expedition is covertly aided and is thus much better supplied/equipped/manned and able to score a few key victories over the Mexican forces. Things are then allowed to get out of hand resulting in "outrages" that are used to make the case for US intervention. The resulting bill barely gets by the Senate and House due heavily to Know-Nothing support. The resulting war is decently short, and though it is a handy US victory, it is deeply unpopular in the North. The Peace gives much of North-west Mexico to the US and takes the place of TTL's Gadsden purchase.
Of course the Democrats get hammered for this next election as the mid-terms bring back an even more polarized House of Representatives and begin to shift the balance of the Senate. The Know-Nothing movement is all but annihilated due to it's vacillating support for slavery and the unified front of the Whigs/Free Soilers, dominates the House of Representatives.
Thoughts?