The wiki page specifically says that it’s highly criticized:

Several political scientists, psychologists, and sociologists have criticized the horseshoe theory. […] political scientists do not appear to support this notion, and instances of peer-reviewed research on the subject are scarce. Existing studies and comprehensive reviews often find only limited support and only under certain conditions; they generally contradict the theory’s central premises.

Just because it has a wiki page does not mean it’s a valid theory.

https://en.wikipedia.org/wiki/Flat_Earth_Theory

That’s kind of true, but it depends. Most states don’t have laws requiring closed primaries and allows the party to choose which voters to include. However, in general the Republican party tends to have closed primaries when possible.

They are closed in Pennsylvania, though.

https://www.ncsl.org/elections-and-campaigns/state-primary-election-types

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