Final answer:
Theodore Roosevelt said the quote, expressing his disappointment in not being able to make a significant impact on the nation.
Step-by-step explanation:
The quote, 'I aimed at the nation's heart, but hit it in the stomach,' was said by Theodore Roosevelt.
This quote refers to an incident in Milwaukee, where Roosevelt was campaigning and was shot in the chest by a fanatic. Though he survived the assassination attempt, the quote reflects his disappointment in not being able to make a significant impact on the nation.