In the 16th Lok Sabha, Sonowal was elected from the Lakhimpur constituency. Before this, he served as BJP President from Assam. From 1992 to 1999, he was also the president of the All Assam Students Union.
Born in the Dibrugarh district, Sonowal was elected as MLA from the Moran constituency of Assam. Later in 2004, he was elected in Lok Sabha from the Dibrugarh constituency.
Before joining the BJP, Sonowal resigned from all executive posts held in the AGP due to discontent as the party was trying to make an alliance with another party that was against the removal of the IMDT Act.
The Illegal Migrants (Determination by Tribunal ) or IMDT Act is an instrument to detect illegal immigrants and expel them from Assam. It is only applied in Assam whereas, the Foreigners Act applies to the rest of India. Under IMDT Act, the decision of proving the citizenship of a suspected illegal immigrant rests on the accuser or the police. But, in the other one, the onus lies with the person to prove his legitimacy. This caused difficulty in the removal of illegal immigrants.
In order to wipe off illegal immigrants out of Assam, the 58-year-old politician took up the issue with the Supreme Court. In 2005, the apex court termed IMDT Act unconstitutional and said the Bangladeshi nationals who have crossed the border illegally have no legal right to stay in India and will be deported.
Ahead of the Assam Assembly elections, the Chief Minister said that if the BJP is voted to power, it would bring an error-free National Register of Citizens and ensure that no "illegal migrant" is staying in the state. He also informed that the party has been working to safeguard the interests of the indigenous people of Assam.