The fact that Singh had never won a Lok Sabha seat, his considerable goodwill and Sonia Gandhi's nomination won him the support of the UPA allies and the Left Front.
After the 2004 general elections, the Indian National Congress stunned the incumbent National Democratic Alliance (NDA) by becoming the political party with the single largest number of seats in the Lok Sabha. In a surprise move, United Progressive Alliance (UPA) Chairperson Sonia Gandhi declared Manmohan Singh, a technocrat,
Singh was first elected to the upper house of the Indian Parliament, the Rajya Sabha, in 1995 and was re-elected in 2001 and 2007. From 1998 to 2004, while the National Democratic Alliance was in power, Singh was the Leader of the Opposition in the Rajya Sabha. In 1999, he ran for the Lok Sabha from South Delhi but was unable to win the seat.