There is no question that the Age of Enlightenment was the precusor to Romanticism, but, perhaps, Transcendentalism acted as a transition of thought as the Romantic movement burgeoned. Certainly, the concept of the spiritualism of man and the power of intuition influenced much Romantic thought. An anonymous pamphlet declared,
Transcendentalism....asserts that man has something besides the body of flesh, a spiritual body with senses to perceive what is true, and right and beautiful,...and a natural love for these, as a body for its food.
Emerson called this spirit the Over-Soul. Like the Transcendentalists, the Romantics came to believe that the spirit and imagination was able to apprehend truths that the rational mind could not reach. They, like the Transcendentalists, moved from the limits of rational thought toward the integrity of nature and the freedom of the imagination.
I would say that one of the major precursors to the Romantic movement was the Age of Enlightenment. This period provided the blueprint for everything that the Romantics defined themselves against. For example, the Age of Enlightenment praised and vaulted science above all. The Romantics took the opposite view, suggesting that science's ordained structure and constant predictability robbed the essence of the individual that lay in wonderment, amazement, and emotion. Whereas the Age of Enlightenment sought to reduce consciousness to one or a series of variables in an equation that answered consciousness, the Romantics praised mystery, the unknown, and felt that the human psyche was far too divergent to be reduced to anything. The construct of the Industrial Revolution and the acquisition of wealth, along with the growth of cities and the expansion of factory life was seen with disdain by the Romantics, who encouraged individuality, the breaking of conformist bonds, and a reverence of nature. I think that the Age of Enlightenment became a precursor to Romanticism because it provided a base from which to formulate a response to it.