She has been reaching out to UPA allies and non-UPA parties to invite them to the meeting.
She has written to DMK chief MK Stalin, Janata Dal (Secular), Nationalist Congress Party, and Uttar Pradesh opposition alliance partners, Mayawati and Akhilesh Yadav, among others.
Sonia to be prepared for a 'less than decisive' verdict
Sonia reportedly took the lead in consolidating opposition parties to be prepared for the possibility of a "less than decisive" election result.
The Congress leader has maintained a low profile in this General Election, but her children Rahul and Priyanka Gandhi have been campaigning against the BJP aggressively.
But as the day of election results approaches, Congress requires every possible ally on board.
Several senior opposition leaders likely to attend
Meanwhile, several popular leaders, including Andhra Pradesh CM Chandrababu Naidu and Sharad Pawar, are likely to attend the opposition meeting. Congress reportedly reached out to Telangana CM Chandrasekhara Rao (TRS), Odisha CM Naveen Patnaik (BJD), and Jaganmohan Reddy's YSR Congress.
Congress won't make an issue if PM isn't from party
Senior Congress leader Ghulam Nabi Azad, meanwhile, said that they will join hands with other parties and also indicated that the party wouldn't "make an issue" if the Prime Minister is not from Congress.
"We are not going to make an issue that we (Congress) will not let anyone else become the Prime Minister if it is not offered to us (Congress)," he said.
Congress wants the support of opposition parties
According to sources, the Congress "doesn't want a repeat of Goa" where the party couldn't form the government despite being the largest party as the BJP came to power with the help of smaller parties and Independents.
Congress wants to make sure "no time is wasted" on 23 May; so, it is inviting opposition parties to the Delhi meeting on results day, sources said.
These three key opposition leaders wouldn't attend the meeting
However, key opposition leaders, West Bengal CM Mamata Banerjee (TMC), BSP chief Mayawati, and Akhilesh Yadav (SP), are likely to skip the 23 May meeting. Mayawati reportedly told AP CM Naidu that there was no need for any meeting before the election results are declared.