This is the end result of decades of gerrymandering.
Maybe a good metric would be the ratio of primary ballots cast to general election ballots cast. If a small number of extremist primary voters are de facto selecting the candidate that will win the general and get the "safe seat", that suggests an opportunity for disruption by being a high profile 3rd party candidate who's just a bit more moderate than candidates the extremists select (more appealing to the median voter).
Another idea is to reach out to a bunch of independent candidates and try to recruit them for your new party to work towards critical mass.
Look at the Tea Party and the DSA; they have enough motivated bases to form parties of their own, but their strategy is to co-opt and purify the existing mainstream parties. Equivalent movements in Spain decided to make their own parties and Podemos and Vox became the next two largest parties in Spanish elections.
It's not like party establishments haven't had candidates lose despite all odds. And Independents have had good streaks at lower offices (e.g. Bernie Sanders, Michael Bloomberg)