Localist parties can probably win as well. I think there are some observations that can be made from UK elections, which also use first-past-the-post.

• Local political parties can win. The Scottish National Party did well in Scotland for several years (until their poll numbers collapsed after their former leader quit and got arrested)
• It makes more sense for small parties to pour all their resources into contesting a small number of seats than to contest and lose a large number of seats. The UK Green Party spent a lot of effort to get their leader elected to Parliament in the Brighton Pavillion constituency.
• Local representation matters. When your party controls several seats on a local council or devolved assembly, they have more chances to gain visibility or even govern. US parties should spend a lot more effort on state legislative races than the presidential one.
• Vote-splitting is less of a concern when one ideology is already overwhelmingly dominant in a region. That is a good region to try to win. For example, the DC Statehood Green Party is the second-largest political party in Washington, D.C. because the DC Republican Party is tiny and terrible (polls in the single digits). That’s a good place to try to win some seats.