First of all, it's Mangum, not Magnum. (But everyone gets that wrong at times--even me...)
Second, here is an old soc.history.what-if post of mine on this subject:
***
So what are the consequences of President (or Acting President? [1])
Mangum? He had been a loyal supporter of Clay and his economic program
against Tyler, and had joined other congressional Whigs in reading Tyler
out of the party. (Mangum called Tyler an "imbecile" and "drunken with
vanity."
http://ufdc.ufl.edu/AA00003610/00001/325j It is indeed
remarkable how overwhelmingly the Whigs--including many southern Whigs who
had originally joined the party out of a concern for states' rights--
united in defense of Clay's nationalistic program. It was really the
first time the Whigs had shown themselves to be a disciplined national
party with a real platform.) However, the Democrats had won control of
the House in the mid-term elections, so there was no question of enacting
the Whig economic program in 1844. The real question, of course, is
Texas. I have just looked up a dissertation on Mangum's life which
summarizes his views as follows:
"Before 1844, North Carolina Whigs generally favored the idea of bringing
Texas into the Union. When Secretary of State Abel Upshur first broached
the subject to Mangum in January 1844, the President Pro Tempore expressed
his regret that the bill would be credited to Tyler instead of Clay.
Mangum had no philosophical objections to the idea, only to the fact that
Tyler would reap the benefits of it. After the Raleigh letter, however,
[in which Clay announced his opposition to the treaty of annexation] he
led the Tar Heel Whigs in denouncing annexation. Only then did they voice
their concerns for the country's honor, the risk of war, or the threat to
cotton prices brought on by overexpansion. In the end it was Clay's
desire to maintain good ties with his northern allies and his wish to see
Tyler fail, not an abiding concern for Mexico's sovereignty or America's
honor, which prompted him to declare against annexation. Similarly,
Mangum and most southern Whigs voted to reject the treaty out of loyalty
to Clay, not to uphold a sacred principle or avoid war." Joseph Conan
Thompson, *Willie Person Mangum: Politics and Pragmatism in the Age of
Jackson* pp. 336-7.
http://ufdc.ufl.edu/AA00003610/00001/345j
I think that the Whigs, who for two years had settled on Clay as their
presidential choice for 1844, would still favor him for the nomination
even if Mangum became (Acting?) President. They would view Mangum as a
good man, but an accidental president, who should make way for his friend
Clay when it came to the nomination for a full term. And certainly they
would oppose dividing the Whig party by persisting with the annexation
treaty, which was violently opposed by northern Whigs. This is
particularly true because southern Whigs, including Clay and Mangum
himself, underestimated the appeal of Texas annexation in their section.
(The Democrats, Mangum wrote, "count much on Texas & its excitements.
They will be mistaken I think."
http://ufdc.ufl.edu/AA00003610/00001/344j
Of course part of the reason for this confidence was that Mangum, like
most Whigs, at first underestimated Polk, "Who Is James K. Polk?" being
the famous Whig slogan.) In any event, the treaty cannot pass the Senate
over Clay's opposition (and AFAIK this was before annexation by joint
resolution was seriously considered). So my guess is that Mangum, while
not repudiating eventual annexation in principle, finds some excuse to
withdraw the treaty.
In November, the people choose between Clay and the Democratic nominee.
Who will that be? I think Van Buren's chances of getting the nomination
are better in this ATL than in OTL, because (a) Democrats will not have to
fear that nominating Van Buren will lead to a Tyler third party immediate-
annexationist candidacy that could attract many southern Democrats [2];
and (b) Tyler and Calhoun will not have the "bully pulpits" of the
Presidency and the Secretary of State's office to terrify Southerners
about the Evil British Plot to Abolitionize Texas. Still, Andrew Jackson
and others will insist on an annexationist candidate, and in any event it
is possible that Texas was just an excuse for some anti-Van Buren
Democrats. (A good many worried whether he could win, given his
association in the public mind with the hard times that followed the Panic
of 1837.)
If, as I think, Van Buren is the nominee (after all, even in OTL the vote
to impose a two-thirds requirement, which doomed Van Buren's chances,
passed only narrowly) IMO Clay wins. If it's still Polk, I am less
certain. With the Texas issue even slightly less intense, Clay might
carry Louisiana (which he probably did even in OTL, only to be counted out
by Democratic fraud in Plaquemines Parish) and Georgia, but those by
themselves will not be enough unless he carries some northern state he
lost in OTL. New York of course is the key. With less excitement about
Texas, Clay might carry the state by getting votes that went to the
Liberty Party in OTL. (BTW, one thing I wonder is whether Mangum as
President could do anything to crack down on the illegal naturalizations
in New York which did so much to hurt Clay there. [3])
These are just some preliminary thoughts; I want to read Thompson's
dissertation on Mangum
http://ufdc.ufl.edu/AA00003610/00001/1j more
thoroughly before commenting further...
[1] Tyler's right to call himself President rather than Acting President
was disputed by many Whigs. Even if one thinks it is valid, the case
might be different for a President Pro Tempore or other officer who
becomes POTUS under the Presidential Succession Act, given that Article
II, section one of the Constitution specifically says that in the event of
the removal, death, resignation, or inablity of both the President and
Vice President "Congress may by law...[declare] what Officer shall then
*act* as President..."
[2] But might Calhoun become a third-party candidate in such a case? He
had long believed that on matters vital to the South, there was no
difference between Clay and Van Buren, contemptuously but accurately
anticipating the 1844 positions of the two as early as 1838: "the two
prominent candidates Mr. Van Buren and Mr. Clay naturally come together on
all questions on which the North and South come into conflict...They of
course dread all conflicting questions between the two sections, and do
their best to prevent them from coming up, and when up to evade them."
(Quoted in Michael Holt, *The Rise and Fall of the American Whig Party*,
pp. 171-2.)
[3] I discuss the role of fraud in the 1844 election at
http://groups.google.com/group/soc.history.what-if/msg/aeabb01fe6f6c411
