"Manifesto" or not, you are assigning vastly more importance to Croly's book than it deserves, at least among Republicans.
So, you rightly point out that then ex-President Teddy Roosevelt adopted Croly's term "New Nationalism" for TR's 1912 Bull Moose Progressive Party campaign -- an election that Roosevelt lost.
What you don't mention is that already in 1914, Roosevelt split away from Croly over several issues, including:
Finally, a reminder of perspectives -- under Teddy Roosevelt, the US Federal spending averaged 2.3% of GDP, while today it is more than ten times that amount, and no politician today, regardless of how conservative they are, has ever proposed returning federal government to what it was under Teddy Roosevelt.
So, your claims that TR was just a wild-eyed radical progressive are losing all sight of the reality, then versus now.
""Manifesto" or not, you are assigning vastly more importance to Croly's book than it deserves, at least among Republicans."
No. The entire phrase "New Nationalism" was on loan from Croly, and the entire Progressive Party was mainly dissident former Republicans. Hiram Johnson, the veep was also a former republican. Albert J. Beveridge, Gifford Pinchot, Joseph M. Dixon, and many more. You don't even need to stay within the sphere of elected officials. Sculptor Gutzon Borglum was a republican turned hard-core big-government progressive.
If TR would've gotten his way, you and I would be registered Democrats today bashing republicans like Joe Biden and Barack Obama - but it didn't turn out that way as Wilson managed to concentrate progressivism in the Democrats instead. That's just how the history turned out.
I know you don't know these things and someone needs to tell you. I know you'll reject it because you don't like inconvenient facts but you can't say you weren't told at least once.
And yes, because Theodore Roosevelt went out there and told them "you gotta read this book from Croly" so many of them would have listened.
It's not like Roosevelt did this one singular time. Roosevelt went out there and recommended Croly's second book "Progressive Democracy" as well. He was the leader of the Croly revival.
Theodore Roosevelt was the leader of big government in his day. That's simply the fact.