After three years of riding on the circuit, Trumbull was elected, in 1840, a member of the lower house of the state legislature from St. Clair County. In politics he was a Democrat as was his father before him. This was the twelfth general assembly of the state. Among his fellow members were Abraham Lincoln, E. D. Baker, William A. Richardson, John J. Hardin, John. A. McClernand, William H. Bissell, Thomas Drummond, and Joseph Gillespie, all of whom were destined to higher positions. Trumbull was now twenty-seven years of age. He soon attracted notice as a debater. His style of speaking was devoid of ornament, but logical, clear-cut, and dignified, and it bore the stamp of sincerity. He had a well-furnished mind, and was never at loss for words. Nor was he ever intimidated by the number or the prestige of his opponents. He possessed calm intellectual courage, and he never declined a challenge to debate; but his manner toward his opponents was always that of a high-bred gentleman. On the 27th of February, 1841, Stephen A. Douglas, who was Trumbull's senior by six months, resigned the office of secretary of state of Illinois to take a seat on the supreme bench, and Trumbull was appointed to the vacancy. There had been a great commotion in state politics over this office before Trumbull was appointed to it. Under the constitution of the state, the governor had the right to appoint the secretary, but nothing was said in that instrument about the power of removal. Alexander P. Field had been appointed secretary by Governor Edwards in 1828, and had remained in office under Governors Reynolds and Duncan. Originally a strong Jackson man, he was now a Whig. When Governor Carlin (Democrat) was elected in 1838 he decided to make a new appointment, but Field refused to resign and denied the governor's right to remove him. The State Senate sided with Field by refusing to confirm the new appointee, John A. McClernand. After the adjournment of the legislature, the governor reappointed McClernand, who sued out a writ of _quo warranto_ to oust Field. The supreme court, consisting of four members, three of whom were Whigs, decided in favor of Field. The Democrats then determined to reform the judiciary. They passed a bill in the legislature adding five new judges to the supreme bench. "It was," says historian Ford, "confessedly a violent and somewhat revolutionary measure and could never have succeeded except in times of great party excitement." In the mean time Field had retired and the governor had appointed Douglas secretary of state, and Douglas was himself appointed one of the five new members of the supreme court. Accordingly he resigned, after holding the office only two months, and Trumbull was appointed to the vacancy without his own solicitation or desire.