Clay wins in 1844.
No Mexican War.
Texas remains independent, protected by Britain.
Mormons settle in Utah, establish independent state of Deseret.
Gold found in California, causing influx of foreigners. Miners and Californios secede from Mexico, form Republic of California (under British protection).
The US and Britain both claim the Oregon country; Southerners in Congress block all moves to organize Oregon Territory as a potential free state; some conservatives (?) oppose the entire idea of extending the United States to such a remote area. The delay causes settlers there to organize their own government. This leads to a compromise with Britain: both countries renounce their territorial claims in favor of the new Republic of Columbia.
Meanwhile, the slavery issue continues to fester. The Supreme Court rules that neither Congress nor territorial governments may prohibit slavery in territories. Slavery men try to make Kansas a slave state, resorting to force even more than OTL, with the connivance of President Pierce (1857-1862).
(Whigs won in 1848 and 1852; but in 1856 split over slavery, allowing the Democrats to elect Pierce. Northern Whigs formed the abolitionist Liberty Party, which competed with the old-Whig American Party, allowing Pierce to win enough of the North to be elected and re-elected. Pierce died of liver failure, succeeded by VP Robert Toombs.)
In 1863, Toombs engineered the admission of Kansas as a slave state. Going beyond what even Pierce could stomach, he vouches for a grossly fraudulent territorial election imposed at gunpoint by Border Ruffians.
This spawned rage throughout the North. The Liberty Party boomed. In 1864, the Democrats dropped Toombs for Indiana Doughface Jesse Bright, trying to dodge, but it didn't work this time. There was just enough anger for Liberty candidate Salmon Chase to to carry 13 free states, with 149 out of 296 electoral votes. Six of of them were only plurality wins: many Northerners were still reluctant to vote for an outright abolitionist like Chase. Some voted for Free Soil candidate Lyman Trumbull, nominated by a small faction of Liberty moderates.
After Chase won the election, several Southern states began moves to declare secession. Lame-duck President Toombs, a secessionist, began preparing to recognize secession when it was declared - appointing Southern officers to command of all Army posts in the South. When South Carolina declared secession, he "recognized" it and ordered Army troops withdrawn from Charleston; following up with similar moves as other states followed. (Georgia's declaration of secession was set to take effect on 3 March 1865, so that Toombs could serve out the remainder of his term.
One group representing several Upper South states submitted a joint petition to President-elect Chase, demanding that he pledge to uphold a list of pro-slavery policies and principles. When Chasr balked, those states joined in secession, while many Northern men denounced him for provoking.
Upon taking office, Chase was faced with secession as a fait accompli, and little support in the North for a war to defeat secession. He gave in, and followed Toombs' lead. The remaining slave states all declared secession in the next two months.
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So the OTL US ends up divided among seven countries (USA, CSA, Texas, Deseret, California, Columbia, and Kingdom of Hawaii). None of them have the resources to become a "world power".