Well, 1844 was all about Texas and Cass wasn't seen as very bold on that issue, even though he did support annexation. If he were more gung-ho about it, I imagine he'd make different choices than Polk but we'd still get the Mexican-American War.
Cass put more emphasis on Oregon than on Texas--which is natural for someone from a northwestern state like Michigan. And Cass had been an Anglophobe ever since he had witnessed Hull's surrender of Detroit.
So would Cass have insisted on "54 40 or fight"? Would he have avoided the Mexican War to concentrate on Oregon? I discuss this at
https://groups.google.com/d/msg/soc.history.what-if/toUpmIq0zV4/yXx-cQdX1FEJ:
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The most plausible POD here is Lewis Cass getting elected president in
1844 instead of James K. Polk. Cass was a northwestern expansionist, an
Anglophobe, and a strong defender of US rights to "fifty-four forty."
There are, however, at least three problems:
First, it is hard to see Cass winning the nomination, given the hatred the
Van Burenites felt for him. In order to stop Van Buren and have a chance
to be nominated, Cass (and his supporters) had to re-instate the two-
thirds rule. But by doing so, he embittered the Van Burenites so much,
that they were determined that even if Van Buren couldn't get the
nomination, at least they would see that Cass would not. And as they were
over one-third of the delegates, they could and did block him.
Second, I am doubtful that Cass, had he gotten the Democratic nomination,
could have defeated Clay. As a non-slaveholder, Cass would have less
appeal than Polk in the South, and I am not sure he would make up for it
in the North. The hatred of the Van Burenites would almost certainly cost
him New York (which Polk very narrowly carried in OTL) and possibly
Pennsylvania as well. Cass might do beter than Polk did in the Old
Northwest (though even this is doubtful, because Polk in 1844 appeared to
be a good all-Oregon man) but the only state in this area that Polk lost
was Ohio
http://psephos.adam-carr.net/countries/u/usa/pres/1844.txt and even if
Cass carried Ohio, this could not make up for losing narrow Polk states
like New York, Pennsylvania, Louisiana, and Georgia.
Third, let's suppose that Cass is elected. It was one thing for Cass *as
senator from Michigan* to make "fifty-four forty" speeches in Congress, to
vote against the Oregon Treaty, and to scoff at the notion that a war with
Britain would be ruinous to the US:
"Happen what may, we can neither be overrun nor conquered. England might
as well attempt to blow up Gibraltar with a squib, as to attempt to subdue
us. I suppose an Englishman never even thinks of that, and I do not know
that I can exhibit in stronger terms its impossibility.
"I might easily spread before the Senate our capacity to annoy a maritime
adversary, and to sweep the British flag from this part of the continent;
but I forbear. What we have twice done in the days of our comparative
weakness, we can repeat and far exceed in these days of our strength.
While, therefore, I do not conceal from myself that a war with England
would temporarily check our progress and lead many evils in its train,
still I have no fear of the issue, and have an abiding confidence that we
shall come out of it, not indeed unharmed, but with all the elements of
our prosperity safe, and with many a glorious achievement written on the
pages of our history..."
http://books.google.com/books?id=BzIFAAAAYAAJ&pg=PA277
But it is another thing to refuse to compromise on Oregon, when you are
president of the United States, representing not a small northwestern
state but the whole country, and in particular when you are the leader of
a party which includes many people far more eager for a war with a weak
Mexico than with a strong Great Britain--especially Southerners who may
not be sure whether Mexican territory will be suitable for slavery but
*know* that British territory will not...
I suppose that problems one and two could be overcome if Van Buren dies in
an accident sometime before 1844. I discuss that at
http://groups.google.com/group/soc.history.what-if/msg/4738a9e15d74098f
(It was entitled "Cass in 1844--Part One" but it seems I never got around
to writing a Part Two.) Problem three would still remain, though: Cass
was a Texas man as well as an Oregon man, and the acquistition of Texas
made war with Mexico likely, and the US could not very well fight Mexico
and Great Britain at once...