> get all the liberal/progressive voters to register as Republicans

Sorry, didn’t quite follow that! You can vote for anyone regardless of who you Registered for? Or, was that suppose to give a misleading signal to Republicans that they have way too many voters? :-)

Sorry, you have to register as a republican so you can vote in the primary. Primaries are frequently the only competitive race in an election.

Also, running as a republican gets past the automatic "reject liberal/democrat" reflex

It depends on the state (in some you can register as a democrat and ask for a republican primary ballot), but I did this so I could vote against George W three times. (If only we could have him again instead of Trump…). You can register for whatever party you want, but some states have early deadlines.

One problem with creating real change with this approach is that the party elites get to decide who are on their ballots.

A while back, Colbert (?) tried to run as a republican and documented all the roadblocks he hit.

To get an idea of how it went, imagine a popular candidate going to a southern plantation to kiss the rings of the great-grandchildren of slave owners.

After deciding there is no personal upside to them, they decide to keep the candidate off the ballot and ask a servant to freshen their mint julep.