To make *part* of California a slave state, you could have the Missouri Compromise line extended to the Pacific Ocean, as Secretary of State Buchanan advocated in 1847. Next to the Calhoun doctrine that the Constitution carried slavery into *all* the territories, this was the most pro-Southern possible resolution of the territorial issue. Even southerners who agreed with Calhoun in theory would accept Missouri Compromise extension in practice, because hardly anyone thought that slavery had much chance of going into territories north of 36°30′. As to whether it might go into the portion of the Mexican Cession south of that line, there were more doubts; but even if it would not take root there, there was still the *precedent* extension would set. At that time most people thought that eventually the US would annex additional territory in Mexico, Central America, and the Caribbean; and extension would set the precedent that all such territory would be slave territory.
For that reason, while extension might pass the Senate, it would have a very hard time in the House, where the North had a majority. You might say that sufficient pressure and patronage would persuade enough northern Democratic "doughfaces" to vote for it to get it through the House, as with Kansas-Nebraska in 1854. But one difference is that there were a lot more northern Whigs (and fewer Democrats) in the House in 1848 than there were in 1854. Anyway, after the Gold Rush and the influx of a mostly northern population into California, the cause of partititon was probably doomed--the predominant sentiment in California was for it to be one state, and without slavery. Even most southerners recognized in 1850 that California was probably lost; what they wanted was to get as much compensation for it as possible.
There was a movement in California in 1859 to detach the southern counties from the state and make them a "Territory of Colorado." This has often been seen simply as a pro-slavery move, because any such territory would be slave territory under the *Dred Scott* decision (and the idea of division was backed by the pro-Southern "Chivalry" wing of the state Democratic party) but the Hispanic southern Californian politicians who promoted the idea were probably less interested in introducing slavery than in augmenting their own political power and not having to pay state taxes, from which they felt they were getting very little benefit. There was of course little chance that the US House of Representatives, controlled by Republicans by this time, would assent to such a division...