How far right will Republicans need to go before it starts truly costing them elections?
In 2016, Donald Trump's electoral win was a massive symbolic defeat, not only to the left, but also the center and even some of the center right. This was because not only was he an outsider with little or no political experience before, but because he had taken certain things about the Republicans that were already there implicitly and taken them a little bit higher. This was most evident in the "Build The Wall" concept which was designed, really, to keep brown Hispanics out because they do not want dark skinned people who speak different languages to move here even as they fill vital positions in agriculture and construction in particular.
I feel like they might have already reached this point ideologically. Even though many/most true far-right candidates are losing, right-wing primary challengers such as Ken Paxton, many of whom were endorsed by Trump himself, are winning. And when Trump's endorsed candidates lose in Republican primaries, it is usually to another candidate that is also very Trumpist sometimes even more so than the opponent.
Young Republicans are actually a lot more likely to hold opinions classified as right-wing extremism. Polling of Republican voters including Republican primary electorates, shows that the youth within the party are significantly more likely to explicitly (at least to themselves) have views like "America should be a predominantly white nation" or "Democrats are importing people to change our demographics to change our culture and tilt elections in their favor".
So, these losses will likely come from elders. In fact, they probably already started. Exit poll comparison from CNN says that Trump carried elderly voters (those 65+ years old) by 5% in 2020. But in 2024, it went to 1%, making it the only major demographic to shift Democratic that cycle. In 2026 and beyond, it would not be surprising to see Democrats win every generation except Gen X and the "Silent Generation".