I had been told this my entire life, and then one day I decided to look at this claim.
What I see is that this claim doesn't stand up to facts and reason. It was literally impossible to grow cotton in the territories, and nothing else would pay the slaveowner enough to make it worth the cost of keeping a slave.
(Slaves cost around $1,000.00 in 1860 money which is approximately $100,000.00 in modern money. )
Further proof is the Wikipedia entry on "New Mexico Territory" where it explicitly says there weren't a dozen slaves in the entire (stretching from Texas to California) territory.
So what if this "expansion of slavery" claim was just a lie?
That is now the model under which I operate in understanding what happened.
in 1860, in the New Mexico Territory, an area which encompassed the area presently occupied by the States of New Mexico and Arizona, that there were a grand total of 22 slaves, only 12 of whom were actually domiciled there. If the South intended to be a “Slave Power,” spreading its labor system across the entire continent, it was doing a pretty poor job of it. Commenting on this fact, an English publication in 1861 said, “When, therefore, so little pains are taken to propagate slavery outside the circle of the existing slave states, it cannot be that the extension of slavery is desired by the South on social or commercial grounds directly, and still less from any love for the thing itself for its own sake. But the value of New Mexico and Arizona politically is very great! In the Senate they would count as 4 votes with the South or with the North according as they ranked in the category of slave holding or Free soil states”.
They noticed the obvious. What the Southern states wanted was more seats in the Senate so as to block even more harmful legislation like higher tariffs that would bleed even more money out of their pockets. Notice how when they left, they were just fine with not staking a claim to any of the western territories. They no longer needed seats in the US Senate to protect themselves.
“What do you propose, gentlemen of the free soil party? Do you propose to better the condition of the slave? Not at all. What then do you propose? You say you are opposed to the expansion of slavery. Is the slave to be benefited by it? Not at all. What then do you propose? It is not humanity that influences you in the position which you now occupy before the country. It is that you may have an opportunity of cheating us that you want to limit slave territory within circumscribed bounds. It is that you may have a majority in the Congress of the United States and convert the government into an engine of Northern aggrandizement. It is that your section may grow in power and prosperity upon treasures unjustly taken from the South, like the vampire bloated and gorged with the blood which it has secretly sucked from its victim. You desire to weaken the political power of the Southern states, - and why? Because you want, by an unjust system of legislation, to promote the industry of the New England States, at the expense of the people of the South and their industry.” - US Senator Jefferson Davis 1860