I'll repeat an old soc.history.what-if post of mine:
***
"Let me try to suggest one [way of admitting Utah with polygamy] that
wouldn't involve civil war. It has to do with Zachary Taylor's idea in
1849 that Utah be (temporarily) included in a larger California. I get
the following mostly from Michael Holt's recent *The Rise and Fall of the
American Whig Party* (pp. 437-9):
"It is widely recognized by historians that Taylor wanted to avoid the
Union-threatening issue of the Wilmot Proviso by simply admitting the
newly acquired Mexican cession territories as states, thus skipping the
territorial stage altogether. (That they would probably be free states
did not bother him, although he was a slaveholder; he thought slavery
neither needed to expand nor had any prospect of taking root in the
Southwest; the important thing was to spare the South the humiliation of
a Congressional condemnation of slavery in the territories.) His efforts
to get California and New Mexico admitted are well-known. Less attention
has been paid to his efforts regarding Utah (or "Deseret").
"Taylor had sent Savannah Whig Congressman Thomas Butler King by sea to
California to urge its residents to form a civil government; James S.
Calhoun, whom Taylor had sent as Indian agent to Santa Fe, was urging the
same thing of New Mexicans. To make sure that *none* of the Mexican
Cession territory was left out, he also sent Missourian John Wilson as
Indian agent to Utah and instructed Wilson to persuade the Mormons to
join the Union as part of the new state of California. If the state
proved too large to manage, as it undoubtedly would have, Taylor
suggested, it could later be divided.
"While Wilson would travel overland and King by sea, Taylor expected the
two men to act in tandem. If Wilson got the Mormons' consent to this
proposal, he was to proceed west with Mormon representatives to San
Francisco, so that Mormons could attend the Califronia constitutional
convention King had arranged and Utah could be included within the
boundaries of the new state of California. King, meanwhile, was to delay
proceedings in California until he heard from Wilson and then persuade
Californians to include Deseret in their new state.
"Wilson did in fact succeed in persuading Brigham Young to go along with
this plan, and several Mormons, expecting to serve as delegates to the
California constitutional convention, accompanied Wilson on the arduous
trek across the Sierra Nevada for the planned rendezvous with King. But
by the time they got to San Francisco (June 4) it was too late: on the
previous day, at Monterey, Brevet Major General Benet Riley, the military
governor of California had issued a call for a constitutional convention
to meet September 1. Ignoring the instructions to await word from Wilson
about the Mormons' intentions, King toured Califronia with Riley, urging
the residents to apply immediately for statehood. In September the
convention wrote and forwarded to Washington a constitution which
prohibited slavery from the new state and claimed its current boundaries.
The furious Wilson charged that King had violated his instructions because
of his eagerness to secure one of California's Senate seats. (King denied
that his mission and Wilson's were related.) Holt concludes that 'Whoever
was at fault, an extraordinary opportunity had been lost.' (p. 439)
"So: what if King had obeyed his instructions, the Mormons were included
in the Constitutional Convention, and some arrangement was worked out in
the new California constitution by which Utah would be included in
California but would *automatically* split off as a new state--without
the requirement of any additional Congressional action, once Congress had
admitted California--by a certain date or upon attaining a certain
population? To be sure, this means that the South would have to consent
to still another free state, but since most Southerners didn't expect
slavery to take root in Utah anyway, it is possible that just enough
Southern Whigs would vote with the North to admit the greater-California-
soon-to-be-split-into-two-states. (At least they might do so if, as in
the Compromise of 1850 in OTL, sufficent concessions were made to the
South on other issues.) Congress would thus bind itself--at a time when
polygamy in Utah was not yet the major national issue it would be later in
the 1850's--to Utah statehood.
"This is the only plausible scenario I can think of for Utah to be
admitted to the Union with polygamy allowed and without civil war as an
inducement. (I'll admit it has problems--not only in getting
Congressional approval, but also perhaps that the Californians were
dead-set against including the Mormons even temporarily.)"
See
http://groups.google.com/group/soc.history.what-if/msg/77f23cdf1fa88c1d
for my answers to two criticisms of this scenario: (1) that additional
action by Congress would still be required (my answer is essentiaily that
there is no reason that Congress cannot provide for statehood for a given
area *in advance*, to be automatically granted at a certain time or when
certain conditions like sufficient population have been met) and (2) that
Utah did not have enough people for a state: "In 1856, the Utah
legislature took a census which claimed 76,335 Utahans. This was
doubtless much too high a figure---Nels Anderson in Chapter 6 of *Desert
Saints: The Mormon Frontier in Utah* (University of Chicago Press 1942)
suggested they may have counted Mormons in Europe who merely intended to
move to Utah at some time (just as in 1849 the petition for statehood
contained names of Saints who still lived in Iowa). OTOH, he adds that
the 1860 US Census figure (40,295) may have been a short count, since the
counting was done by Gentiles. I do not think that most Americans would
simply accept the Mormons' word about the population of Utah; but for the
sake of (supposedly) ending the slavery-in-the-territories question they
might have been willing not to stress the population issue too much in
1850 and assume (or pretend to believe) that within a few years Utah would
have enough people." (Later in that thread, Doug Hoff remarked "IIRC,
Lincoln and Congress winked at Nevada's population deficit to get that
state into the Union in time for the 1864 election. I think it could be
done in 1850 as well.")
See also "Larger Than Texas: Proposals to Combine California and Mormon
Deseret as One State," by Edward Leo Lyman (originally published in
*California History,* vol. 80, no. 1, Spring 2001):
"From its inception, the Taylor-Wilson statehood initiative proposed only
a temporary union. The two distinct areas were to be bound by a common
constitution and name, but there was to be a specific stipulation for
dissolution in 1851. Brigham Young discounted the weight given to the
vastly superior population of California, arguing that most people were
only temporary gold seekers and that the lack of families proved their
transience. On the other hand, he estimated Mormondom's population at
fifteen thousand and predicted that it would grow to over seventy-five
thousand by the beginning of the year of division, 1851--more than
requisite for separate statehood. The Great Salt Lake area population
increase was indeed large, but the economic base to support it did not yet
fully exist. Young was overly optimistic. Still, the church leader
disclosed expectation that before his domain could be permanently
consolidated with the Pacific Coast province, '1851 will arrive and the
yoke [of the unified state] will be broken. Thus while [the] government is
using us to save the nation, we are using them to save ourselves.' Young
emphatically cautioned Lyman that 'there must be no doubt of our being a
[separate] state at the commencement of '51, without any further action of
congess.'17"
http://web.archive.org/web/20060508195126/http://www.wemweb.com/arduous-road/larger_than_texas.html