Ghulam Nabi Azad said regional party leaders joining Congress as they want to challenge BJP's ideology.
In a boost to the Congress ahead of assembly elections in Haryana, an Independent MLA and four Indian National Lok Dal leaders joined the party on Sunday in the presence of senior leaders.
Indian National Lok Dal (INLD) leaders Ashok Arora, Subhash Goyal, Pradeep Chaudhary and Gaganjit Sandhu and Independent MLA Jai Prakash joined the Congress at the party headquarters in New Delhi.
Ashok Arora, a former minister, was the INLD president for over a decade.
Subhash Goyal is a former minister and Chaudhary a former MLA.
Gaganjit Sandhu is the son of INLD's former minister Jaswinder Singh Sandhu.
Jai Prakash is a former Congress leader and has been a former union minister too.
He has been a three-time MP and is currently an Independent legislator.
Congress general secretary in charge of party affairs in Haryana Ghulam Nabi Azad, along with former Union Minister and Haryana Pradesh Congress Committee chief Kumari Selja and Congress legislature party leader and former chief minister Bhupinder Singh Hooda, welcomed the leaders into the Congress-fold.
Mr Azad said Ms Selja and Mr Hooda have started visiting various constituencies and have started with Gurgaon and Faridabad assembly segments as part of the party's outreach initiative for the upcoming Assembly elections.
"Regional parties which do not toe BJP's line are getting weak and their existence is under threat. Realising that only Congress party can throw a stiff challenge to the BJP, leaders of such parties have started to come under the Congress fold," Mr Azad said.
"Despite difference in opinion, there are similarities in our ideologies because of which we have come together," he added as he thanked them for their faith in leadership of Sonia Gandhi and in the ideology of the Congress party.
"Had the regional parties, which in the past weakened the Congress, been strong, then the BJP would not have risen to this level," he said.
Mr Hooda said that the party would be strengthened with the joining of these leaders