Lee was intended to start his political career in 1990, but promised with other members including Woo Sang-ho, served as the Vice President of ANUSR under Lee, to not do it for 10 years in order to "not ruin the meaning of student movements". He, along with Woo, were brought to the Millennium Democratic Party by Kim Dae-jungin 1999. He ran as the MP for Guro 1st constituency, but lost to Kim Ki-bae of the Grand National Party. He could be elected in 2004 election under the banner of the Uri Party, the splinter group of the MDP. For the first term as an MP, he was a member of Committee of Education and Public Administration and Security Committee of the National Assembly. He lost to Lee Beom-rae in 2008 election, but continued his political career. In 2010, he ran as the party president of the Democratic Party but lost to Sohn Hak-kyu. Instead, he became one of the Vice Presidents. He also ran for the presidency of the newly-formed Democratic Unionist Party, but lost to Han Myung-sook and elected as a Vice President. Lee came back as an MP in 2012 election and worked for Strategy and Finance Committee, Environment and Labour Committee and Gender Equality and Family Committee. He ran again for the presidency of the New Politics Alliance for Democracy in 2015, but came as 3rd and lost behind of Moon Jae-in and Park Jie-won. He was re-elected in 2016 election. On 21 April 2019, Lee declared to run as the parliamentary leader of the Democratic Party of Korea. He was endorsed by National Alliance for Democracy and Peace, The Better Future and The Owls Group within the party. On 8 May, he gained 54 votes and defeated Kim Tae-nyeon who gained 37 votes and Roh Woong-rae gained 34 votes in the first round. Since no one gained the majority, Lee and Kim went to the second round in the same day, where Lee gained 76 votes and defeated Kim with 49 votes. Lee was immediately elected as the new parliamentary leader of the party, replacing the incumbent Hong Young-pyo. On 3 July 2020, Lee was nominated as the Minister of Unification, following the resignation of Kim Yeon-chul due to North Korea's destruction of the Inter-Korean Liaison Office in June. He was officially appointed to the position on 27 July.
Political views
Ideologically, Lee is regarded as a left-wing in Democrats. During an interview with Park Mi-sook of Moonthly Joongang on 17 April 2013, Park asked Lee that Democrats failed as it has moved to left. Lee replied: "Then should the Democratic Party be a conservative party?" He also added that there's no centrism in politics, by providing an example: "What is the centre between reducing and increasing the temporary employees?" However, during the parliamentary leader election campaign, he declared himself to move his position from left to centre.