It always falls down because the end-goal of ones "rationalism" always has to be determined by a set of values.
If you want to form a political ideology based on rationality, your very first step will stick you right in the middle of the sticky-icky world of the humanities. 'Hello deontology, my old friend.'
Even definitions there are extremely hazy. "The most good for the most people".
Define "good". Happiness? Economic prosperity? Community? And over what time span?
Define "most". Percentage of people served? Number of people served?
Define "people". Are you counting citizens? Immigrants? Foreigners? Prisoners? People in the future?
You know the joke in the sciences about how everything distills down to mathematics? I would argue that we just as often distill down to philosophy. You have to reckon with a lot of questions which a stats degree can't help you much with.
Isn’t rationalism (as discussed above) more closely related to utilitarianism? Thus rationalism is more of a consequentialist framework than a deontological one?