The issue is too many people don't look at political party plans, they simply adopt a political party as their identity. They are not republican (or democrats!) because GOP propose plan with item 1,2,3, but rather because being republican is part of their identity. Once that happens and facts goes against that political party, it make the brain analyze it as an attack against identity, and use defensive reaction mechanism to protect identity, chief among them wholesale rejection of facts.
This is not only the republican which do that, but republican happens to be associated with right wing / authority / strength as an image (not necessarily a truth just an image) and far more likely currently to have the facts going against them.