Agendawise nails the current John Cornyn situation:
Perhaps in the ruling class it is true that every vulnerable politician gets a challenge. In the ruling class – the establishment GOP and the Democratic Party - elective politics is not so much a science, as Mackowiak surmises, as it is a business. Not so among conservatives, who are 'general interests', not 'special interests'. As such they don’t stand to gain any fortunes or fiefdoms from elective power.Read the whole thing here.
In the conservative ranks political research and infrastructure aren’t business investments so they are therefore less plentiful and less refined, though the gap is beginning to close. In addition, the ruling class tries to destroy the reputations of power-threatening conservatives while they dignify the reputations of power-entrenching establishment politicians. As a result, it is much easier to find people to agree to run for the parties. It takes a special person to run as a conservative – a person who has counted the cost.
The upshot is, vulnerable establishment Republicans go unchallenged all of the time. If nobody files against Cornyn he will only be the latest. Saying all unchallenged establishment Republicans are unchallengable is nothing more than court journalists trying to cast spells.