There has been a lot of talk on Reddit and elsewhere that the parties had switched platforms over time, mostly evidenced by the demographic and geographical makeup of the party constituencies over time. The idea is mostly that the Republicans started off as a big government party, and that the Democrats were the party of small government, particularly in regards to slavery. The crux of the party switch theory involves that the Republicans, while morally correct, exerted the powers of the federal government over the states in order to suppress the spread of slavery. As the Democrats are currently the party of expanding federal power, the theory goes that the parties must have switched at some point. Most left-wingers point to the Nixon administration and the Southern Strategy as the point where the switch started to take place.

It is a convincing argument for many people, but I think the biggest problem with it is an inaccurate framing of the Republican party platform. Instead of being strictly small government, I would contend that the Republicans have always been strict Constitutionalism. This often coincides with smaller government, but it is not always the case.

I recently started going through the documents at https://www.presidency.ucsb.edu/ to look at what the party believed throughout history, and compare it to the views held by conservatives today.

In 1856, the Republican party stated that:

The Constitution confers upon Congress sovereign powers over the Territories of the United States for their government; and that in the exercise of this power, it is both the right and the imperative duty of Congress to prohibit in the Territories those twin relics of barbarism--Polygamy, and Slavery.

That while the Constitution of the United States was ordained and established by the people, in order to "form a more perfect union, establish justice, insure domestic tranquility, provide for the common defense, promote the general welfare, and secure the blessings of liberty," and contain ample provision for the protection of the life, liberty, and property of every citizen, the dearest Constitutional rights of the people of Kansas have been fraudulently and violently taken from them.

They go on to describe various violations of constitutional rights in the pursuit of preventing the spread of slavery to the new territories, notably including the right to bear arms.

How this applies to modern conservative thought is more apparent in the platform of 1860.

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.

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.

As you can see here, the Republican party opposes the use of federal power to enforce policies on the territory of Kansas. They saw it as a violation of territorial sovereignty by pro-slavery forces, and attacks the Buchanan administration for interfering in what should have been an internal dispute among Kansas residents. This still encapsulates the modern conservative idea maintaining the sovereignty of the states, above federal interests.

When the Civil War started, this was probably the largest government interference that the Republican party involved itself in, and is thus the foundation of the party switch theory. However, I refer back to my earlier statement about the nature of the Republican party: They seem to be more strictly constitutional rather than strictly small government. Some of the more egregious acts of the Lincoln administration were still within constitutional bounds. The Habeas Corpus Suspension Act of 1863 is seen as a large government exercise, which it is. However, the suspension was already within constitutional provision during cases of invasion or rebellion, meaning that Lincoln did not have to extend any authority beyond what he already had.

In 1864, the majority of the platform was dealing with finishing the war, and the demand of an unconditional surrender of the Confederate States. However, it is also the first mention of an taxation policy in the campaign platform.

That the National faith, pledged for the redemption of the public debt, must be kept inviolate, and that for this purpose we recommend economy and rigid responsibility in the public expenditures, and a vigorous and just system of taxation; and that it is the duty of every loyal state to sustain the credit and promote the use of the National currency.

This is somewhat vague, as a "vigorous and just" taxation system could mean quite a lot of different things. But when you look at the 1868 platform, it becomes a bit more recognizable to modern Republicans.

It is due to the labor of the nation, that taxation should be equalized and reduced as rapidly as the national faith will permit.

They also seek to improve the economy through private businesses.

That the best policy to diminish our burden of debt, is to so improve our credit that capitalists will seek to loan us money at lower rates of interest than we now pay and must continue to pay so long as repudiation, partial or total, open or covert, is threatened or suspected.

This type of economic rhetoric continues throughout the Republican history in nearly every party platform from hereon out. It's become a staple of American conservatism to want to reduce taxes and regulations, and has been since the very beginning.

In 1872, more of the traditional constitutionalist language appears.

The Republican party proposes to respect the rights reserved by the people to themselves as carefully as the powers delegated by them to the State and to the Federal Government. It disapproves of the resort to unconstitutional laws for the purpose of removing evils, by interference with rights not surrendered by the people to either the State or National Government.

In 1884 They maintain that while the federal government is the most powerful, the states do have rights that should not be impinged upon.

The people of the United States, in their organized capacity, constitute a Nation and not a mere confederacy of States; the National Government is supreme within the sphere of its national duties; but the States have reserved rights which should be faithfully maintained. Each should be guarded with jealous care, so that the harmony of our system of government may be preserved and the Union kept inviolate.

Now there are some interesting things that have parallels to America today, in particular, the issue of immigration.

In 1884, the Republicans opposed the use of Chinese immigrant labor, defining it as such:

The Republican party, having its birth in a hatred of slave labor and a desire that all men may be truly free and equal, is unalterably opposed to placing our workingmen in competition with any form of servile labor, whether at home or abroad. In this spirit, we denounce the importation of contract labor, whether from Europe or Asia, as an offense against the spirit of American institutions; and we pledge ourselves to sustain the present law restricting Chinese immigration, and to provide such further legislation as is necessary to carry out its purposes.

And again in 1888

We declare our hostility to the introduction into this country of foreign contract labor and of Chinese labor, alien to our civilization and constitution; and we demand the rigid enforcement of the existing laws against it, and favor such immediate legislation as will exclude such labor from our shores.

and

We reaffirm the policy of appropriating the public lands of the United States to be homesteads for American citizens and settlers—not aliens—which the Republican party established in 1862 against the persistent opposition of the Democrats in Congress, and which has brought our great Western domain into such magnificent development.

Compare these statements to modern ones about illegal immigration. Modern Republicans see illegal immigrant labor as inherently exploitative, with low wages, and competes with American labor to drive wages down. Modern illegal immigrants have a stereotype of working in servile positions in agriculture and domestic work.

I'd like to do more digging, as this is just a surface level analysis, and does skip over quite a lot of Republican history, but I think it is some good food for thought when it comes to discussing the history of the Republican party, and whether or not it is still the "Party of Lincoln." I think that looking through the history of what the party believes, it can be honestly said that it still stays true to its original intentions.