It's a tough road either way, but remember, Paul managed to hang on to his House seat IN Texas, despite the establishment. The grassroots or 3rd party activists could range from those who are fully independent to those who are (by preference) enrolled in one of the major parties for the purposes of jumping into an open seat situation. The point is to be ready to pounce and hit the ground running from a separate base of resources, than being joined at the hip to statist party A or B.
The reason Paul could function as a free man in Congress, and on his own resources as a presidential candidate in the primaries, is he avoided GOP PAC money by developing his own national mailing list, email list, CFL PAC, and developed his own meetup and grassroots network of Republican and Libertarian liberty activists. Building on that same independent, cooperative model can produce the same results at the local level to support liberty candidates, both in the election, and once in office.