There was no party switch.

Roosevelt was a very maverick type leader. So saying he was "republican" is kind of a stretch. He even set up his own Bull Moose party to run against his hand picked replacement Taft. So he had a lot of quirky approaches to problems. He was pro government and wanted it to be larger. Coolidge was pro business and wanted the government smaller. Over simplification on my part.

As with many things, the answer is complicated. Some initial thoughts: Both parties struggled to address the concentration of wealth and power that had been happening.

"Progressivism" is a term that is applied to people with some significant differences but for this discussion, I'll apply it to those who wanted government to take action to address the concentration of corporate power and wealth.

There were elements of both parties who came to the conclusion that government should take action. This was more true on the Republican side than Democratic, as to this point Republicans had been the party of using government power for public benefit (Homestead Act, Land Grant Colleges, etc). However, the bulk of those with power in both parties tended to be more classically liberal, as you referenced.

Roosevelt supported breaking up "trusts" if they were running contrary to public interest or abusive to their workers and communities. He didn't think that they all were deserving of being broken up. He preferred strong federal regulation to rein them in.

Wilson campaigned on a platform to break the trusts up. If they were "too big," they were too powerful and should be broken up. Smaller entities would provde more competetion and better service, so "classically liberal" result, but powerful government to get there. Once elected, however, his approach was closer to Roosevelt's plan than his campaign promises.

Taft actually initiated more anti-trust cases than Roosevelt, so kind of acted in a way that was close to Wilson's goals (going after what Roosevelt considered to be "good trusts" was a factor in his 3rd party bid. However, with Wilson and Roosevelt staking out progressive positions and wishing to solidify his partisan base that relied on large corporate support, Taft's campaign in 1912 tended to be more conservative or "classically liberal."

In summary, Roosevelt was Progressive in wanting to use federal power to limit corporate concentration, which carried on some parts of the Republican philosophy. Wilson campaigned on a short-term expansion of federal intervention, which was Progressive, whith the goal of limiting central government power in the long run, as Democrats had preferred to this point. Taft did act as a Progressive in the instance of many anti-trust cases, but was philosophically closer to party leaders who wanted to maintain strong ties to businesses. After World War I, Republicans fully reverted to a non-interventionist (although very cosy and at times corrupt) relationship to businesses. By the time Coolidge came to power, he sought to minimize the intervention and coziness with business.

I hope this helps.

I agree with Swarnstadt's explanation. With regard to your comment, what exactly do you mean by "party switch". It's a term that is often thrown around, but one that may not be entirely accurate. I think a more accurate term would be "party rotation". Please clarify.

I don't know if this helps you much but your comparing apples to eggplants lol. Todays limited government republican party would be virtually unrecognizable to Lincolns strong federalist party. Todays more socially liberal Democratic party has next to nothing to do with the democrats of that era. Its my opinion that todays Democrats were really born out of Thaddeus Stevens "radical republicans" todays Republicans were really born during the extreme states rights vs federal government arguments but that is about as much connection as they have to the parties your talking about.

Ah at least you (indirectly) clarified which of the [one] countries in the world you meant in the main body…

The extent to which the old GOP was progressive is greatly exaggerated. Republicans were already the conservative party by the time that Roosevelt came along. Teddy himself was never meant to be the president, it's just that the conservative president William McKinley died.

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But to answer your question. Yes, they did adhere to classical liberalism. American politics before 1900 was just different factions of liberalism competing against each other.

Dude's already on r/usdefaultism. 'Both' parties have we? Didn't realise we're all in the US of A.

The main elements of the shift are that the Democratic Party built, under FDR, a coalition that could and does viably win national and state and local elections while before him, it could only do so in cases like Grover Cleveland and Wilson where the Republicans were infighting to a point a Dem could sneak in through the back door. Both parties had liberal and conservative wings. The Taft Republicans were the prototype of today's conservative Republicans, Roosevelt's wing fell apart by the Reagan era and has been dead for a long time as of the 2000 election.

The Democratic Party went from a sectional and state party that occasionally won national elections to a national party based on a coalition of marginalized groups and Black people starting to switch from GOP to Dem under the Coolidge Administration and Harding Administration. The biggest cause of the switch was Republicans having a majority in Congress and refusing to address lynching, at the time.

Liberalism and classical liberalism are opposite things. A classical liberal believes in limited government and economic freedom and thus is essentially a conservative. Coolidge and Reagan were classical liberals. Nixon and Hoover not so much.

TR was not a progressive, rather he was a populist. The difference between liberals and populists is that populism could agree with either the right or left, depending on the issue, and progressive liberals are unequivocally elitist. Wilson was liberal; very high-minded, so completely out of touch that he more than anyone else caused World War II by having unrealistic expectations for how the world should work.