I dont want to get in the way of more discussion, but this previous response by u/Rittermeister will likely be useful to you.

(1/2)

It's kind of both and neither, so let's dig in.

Political platforms are still around and we sometimes refer to them, though more often we just describe a combination of policies and rhetoric as a platform without reference to a formal document. In the nineteenth century all that happens too, but the documents themselves carry a bit more significance in an era where your contact with the big names of your party is chiefly through print anyway. This is an age when papers would reprint speeches in Congress in full and verbatim. Sometimes they'd even do full runs of state documents related to a controversy.

Furthermore, platform writing is largely a more formal version of one of the most common forms of popular activism of the era, the public meeting. The way these worked was that concerned citizens would get together on a previously-arranged date to hear speeches from respected community leaders, activists, and so forth about what had them worked up. They would vote officers, though this can be a formality since usually it's the guys who set the meeting up, and a business committee. While the speeches went on, to cheers and boos, the business committee would draft a slate of resolutions. These might also have been prepared in advance, but there are cases of negotiation happening in the moment. The business committee then comes out with the resolutions and they're read and voted on. Sometimes they get rejected and this goes on again with revisions. Usually the resolutions include instructions that they be printed in local newspapers and sent along to politicians who ought to take notice. A political platform is drafted at the party convention in a similar fashion, though the stakes are higher and the power blocs often more diverse. Signing off on one is a public commitment that one is expected to uphold, which goes double for politicians and triple for presidents. In an era with fairly strong parties and a somewhat weaker presidency, failure to do your part from the White House can cause a lot of headaches.

Here's the platform Lincoln ran on. Point #1 is just that Republicans are going to keep Republican-ing. Point #2 objects, by implication, to the strongly authoritarian bend of proslavery politics in the form of some general flag-waving about the Declaration of Independence. It can also be read as a ding against slavery itself, but whether or not the "all men" in "all men are created equal" includes black men is a subject on which the party doesn't have a clear consensus. Most Republicans are not abolitionists, even if they are all antislavery, and few white antislavery people are enthusiastic about general claims of black equality. White abolitionists are somewhat better on this front. I'll come back to the distinction between abolitionism and antislavery in a few.

On Point #3 we get more meaty:

we hold in abhorrence all schemes for disunion, come from whatever source they may. And we congratulate the country that no Republican member of Congress has uttered or countenanced the threats of disunion so often made by Democratic members, without rebuke and with applause from their political associates; and we denounce those threats of disunion, in case of a popular overthrow of their ascendency as denying the vital principles of a free government, and as an avowal of contemplated treason, which it is the imperative duty of an indignant people sternly to rebuke and forever silence.

In 1860, this has a clear meaning: Southerners have been shouting for disunion almost any time slavery is challenged, however remotely, since about the ratification of the Constitution and with increasing regularity and volume since the Mexican War. It's not quite a statement that the GOP is going to attack slavery, but they are drawing a line in the sand.

That the maintenance inviolate of the rights of the states, and especially the right of each state to order and control its own domestic institutions according to its own judgment exclusively, is essential to that balance of powers on which the perfection and endurance of our political fabric depends; and we denounce the lawless invasion by armed force of the soil of any state or territory, no matter under what pretext, as among the gravest of crimes.

And this is a different kind of line. It's been accepted by white Americans of all political persuasions since the 1790s that the nation does not have the rightful power to intervene within the bounds of a state where slavery already exists to cause its end. This, as a practical matter, means that the GOP does not propose to challenge slavery where it already exists...with the exception of the District of Columbia and more tenuous situations in the federal territories. Where there is no state, the United States has full powers to do as it will.

That the present Democratic Administration has far exceeded our worst apprehensions, in its measureless subserviency to the exactions of a sectional interest, as especially evinced in its desperate exertions to force the infamous Lecompton Constitution upon the protesting people of Kansas; in construing the personal relations between master and servant to involve an unqualified property in persons; in its attempted enforcement everywhere, on land and sea, through the intervention of Congress and of the Federal Courts of the extreme pretensions of a purely local interest; and in its general and unvarying abuse of the power intrusted to it by a confiding people.

I could talk to you about Kansas for a good while, but what matters here is that Kansas has a legal, proslavery government established by tremendous election fraud and an illegal antislavery government established by actual whites living in Kansas which consequently has the clear support of most Kansans who matter to US political discourse: the white men. The proslavery government is based out of Lecompton and the Buchanan administration is all in on admitting it as the government of Kansas. Even many Democrats -Buchanan was a Dem- think this is a terrible idea, including the Democrat who started the whole mess by repealing the Missouri Compromise: Stephen Douglas.

This also addresses Dred Scott, a decision Republicans believe was not simply wrong, but also greatly exceeded the authority of the Supreme Court -Roger Taney used it to try to establish a complete resolution of the slavery question in slavery's favor by stripping any and all rights from black Americans- and was gotten through in some connivance with the Buchanan administration. It also happened to declare the GOP's signature issue, no slavery in the territories, unconstitutional. It did all this whilst reaching inside the free states, which had passed various laws granting some limited rights to black Americans.

Point #5 is an anti-corruption line that doesn't have much to do with slavery.

That the new dogma that the Constitution, of its own force, carries slavery into any or all of the territories of the United States, is a dangerous political heresy, at variance with the explicit provisions of that instrument itself, with contemporaneous exposition, and with legislative and judicial precedent; is revolutionary in its tendency, and subversive of the peace and harmony of the country.

Here we go. Taney held in Dred Scott that the nation had no constitutional power to enact any kind of slavery ban in the territories. This is something that would have been news to the men in the room at Philadelphia, since they enacted at least a few such bans. Until this point, proslavery radicals had floated the notion but few outside that circle agreed with them. Taney opted to go hard right.

In a way, this is a moot point. The only slavery bans then still active were out in the Pacific Northwest, which was unlikely to have large numbers of enslaved people any time soon. Instead, territorial government was bent on keeping any black people out. The actual issue here is the Missouri Compromise, with its vintage 1820 slavery ban on all the territory of the Louisiana Purchase except for Missouri and points south of its southern border. It's been repealed since 1860, which caused the whole mess in Kansas and led directly to the creation of the Republican party. The GOP wants it back; this is literally why the party exists. If they take Congress and the Presidency, they could repeal the Kansas-Nebraska Act's relevant portions and then stand hard against efforts to legislate slavery back into the territories. Taney told them they couldn't do it, even if they won Congress and the presidency until the end of time. They would need a constitutional amendment, for which they would need a supermajority of the states. That supermajority would then require slave states to sign on, which they would never do. Absent the power to ban slavery in territories, the number of slave states will likely at least keep rough pace with the number of free states. So slavery remains and expands forever.