In 621, he was created the Prince of Wei (衛, different than his later title of 魏) and posthumously adopted into the line of his uncle Li Xuanba (李玄霸), who had died early without issue.
In 636, Li Tai's title was changed to Prince of Wei (魏), and he was made the commandant at Xiang Prefecture (相州, roughly modern Handan, Hebei), but again, he was not sent there, but remained at Chang'an.
When rumors reached him that the high-level officials dishonored Li Tai, he grew angry at them, but relented after the chancellor Wei Zheng pointed out that, in fact, according to Confucian ceremonies, those high-level officials were in fact supposed to be higher in rank than Li Tai.
Meanwhile, Li Tai's staff member Su Xu (蘇勖) suggested to him that, like the talented princes in past dynasties, he should commission a major literary work, so Li Tai commissioned the work Journal of Geography (Kuodi Zhi) and retained the officials Xiao Deyan (蕭德言), Gu Yin (顧胤), Jiang Yaqing (蔣亞卿), and Xie Yan (謝偃) to head the project.
The work was completed in 642, and Emperor Taizong rewarded Li Tai with a large supply of silk.
By 643, Li Chengqian was so fearful that Emperor Taizong was about to depose him that he conspired with a number of officials, including the major general Hou Junji, Emperor Taizong's brother Li Yuanchang (李元昌) the Prince of Han, the imperial guard commander Li Anyan (李安儼), his cousin Zhao Jie (趙節), and Du Chuke's nephew Du He (杜荷, Du Ruhui's son), to overthrow Emperor Taizong.
Emperor Taizong, in shock, convened the senior officials Zhangsun Wuji (Empress Zhangsun's brother), Fang Xuanling, Xiao Yu, and Li Shiji, as well as officials from the supreme court, the legislative bureau, and the examination bureau, to investigate, and the extent of the plot was revealed.
However, Zhangsun Wuji suggested that Li Zhi, also favored by Emperor Taizong, be created crown prince.
In order to persuade Emperor Taizong to create him crown prince, Li Tai told Emperor Taizong that he would agree to kill his own son and make Li Zhi crown prince should he be allowed to inherit the throne.
Emperor Taizong put him under house arrest, and the next day created Li Zhi crown prince.
Several days later, Li Tai was removed from his official posts and reduced in rank to Prince of Donglai.
Emperor Taizong exiled his close associates and reduced Du Chuke to commoner rank.