"He died beloved, revered and mourned by millions of revolutionary fellow workers -- from the mines of Siberia to California, in all parts of Europe and America." -Friedrich Engels on Marx, 1883 funeral speech

Karl Marx's close circle in London certainly thought he had a following in the Western United States at that time. Were they right?

He was certainly not entirely unknown. The most prominent illustration would be the attempt to revive the 'International'associated with Marx under the name of the International Workmen's Association, based in San Francisco circa 1882-83 until around 1888 with ties as far-flung as Topeka, KS, Seattle, WA and Sinaloa, MX. This organization was founded while Karl was on his deathbead, but its architects did get in touch with associates of his in London including his daughter and sons-in-law. This organization sometimes portrayed itself as 're-uniting red and black,' meaning marxian ('scientific') and anarchistic socialists who had split as part of the 'First International' the decade prior. Just as often, though, it leaned into the 'Red' side, favoring Marx while contrasting their 'Red International' with the anarchistic 'Black International' more prominent in places like Chicago.

The Denver Labor Enquirer, the most prominent Western labor paper during the 1880s and for a time the organ of this Western Red International, first mentioned Marx in an obituary shortly after his passing, following that up with a biographical sketch.. Marx,s writings were also occasionally printed in short translations in the 'Enquirer' and the other main publication associated with the Red International, 'San Francisco Truth.'

The Red Internationals were deeply embedded in the networks of the Knights of Labor as well as western Trades Unions and certainly spread the ideas of Marx alongside other favorites of theirs like Henry George and even Peter Kropotkin (whose first US translation appeared in 'Truth'). They certainly had widespread ties in mining camps, and at least one organizer affiliated with both Red and Black Internationals, J Allen Evans, could be seen as something of a 'cowboy'. (See Mark Lause's 'The Great Cowboy Strike: Bullets, Ballots & Class Conflicts in the American West').

But whether one could say Marx specifically was a widely popular figure is a bit harder to answer affirmatively. Setting aside the lack any sort of opinion polling, your EP Goodwin quote does offer a different interpretation to 'Marx was popular.' My own research would certainly support the idea that cowbows and ranchers sometimes engaged in highly politicized discourse characterized by a strong opposition to monopoly, a celebration of hard-working 'producers' and 'real-settlers,' and a bit of irreligiousity sometimes mixed in. In that sense, Goodwin may have been using the well-known figures of Ingersoll, Marx, and Kearney (an infamous figure who was likely more famous among westerners in 1880) as shorthand to illustrate the various themes and slangs common in the west at that time. Some of them knew about a guy named Karl Marx, a small number drew significant inspiration from Marx's writings, but many of his ideas and theories simply had a place in the vernacular of western laborers even without citing him specifically.