See Gary Kornblith on a Clay victory in 1844:
"The destiny of California under a Clay presidency is harder to project with confidence. Even without the advent of war with the United States, Mexico would have sustained its claims to sovereignty only with difficulty. In early 1844 the beleaguered governor of California, Manuel Micheltorena, recommended to his superiors in Mexico City that they consider handing the province over to British creditors rather than let it fall into the hands of American immigrants and californios (Californians of Hispanic descent). 'In August 1844,' wrote David J. Weber, 'a group of californios met secretly with British vice consul James Forbes in Monterey and told him they were ready to drive Micheltorena out of California, declare independence, and ask for British protection.' Without instructions from London, Forbes was stymied, but the rebels nonetheless succeeded in ousting Micheltorena in early 1845. They stopped short of declaring independence, however, and soon divided among themselves. Meanwhile, Americans in California prepared to take matters into their own hands, and in June 1846 they staged the Bear Flag Revolt. 'Even if [the Mexican-American War] had not occurred,' Weber asserted, 'Americans in California had become numerous enough to think they could play the 'Texas game' and win.'
"Whether the discovery of gold in 1848 would have prompted President Clay to show more enthusiasm for annexing California than he did for annexing Texas is hard to know. Fellow Whig (but political rival) Daniel Webster had long hoped to acquire San Francisco and the surrounding area for the United States. Yet Clay was more sensitive than Webster to sectional tensions and to the explosive consequences of adding new territory to the federal domain. As with Texas, Clay might well have preferred strong commercial ties with California to the national and international controversies sparked by annexation. For this reason, he would probably have encouraged California to remain independent so long as it avoided an open alliance with Great Britain or another foreign power. Certainly, the possibility that California could have flourished as a separate nation deserves serious consideration. The historical geographer D.W. Meinig has written, 'Was there ever a region better designed by Nature for separate geopolitical existence than Alta California--a land so distinctive and attractive, set apart by the great unbroken wall of the Sierra Nevada backed by desert wastelands, fronting on the world's greatest ocean, focused on one of the world's most magnificent harbors?'
"Alternatively, under pressure from Democratic expansionists in Congress, a President Clay might have proposed pairing the annexation of Texas and California--a reprise of the Missouri Compromise with its coupling of Missouri and Maine. But that scenario seems less probable than the establishment of an independent California because it presupposes Mexico's peaceful acquiescence, a most unlikely development. In keeping with past policy, Britain would have supported Mexican objections to American annexation (as distinct from Texan or Californian independence) and Clay would have backed away from a war for territorial expansion. His commitment to diplomacy, rather than force of arms, would almost surely have curtailed the country's westward growth for the duration of his presidency..."
https://web.archive.org/web/2017082...te/assets/documents/02_JAH_2003_kornblith.pdf