Naidu alleged that every institution, including CBI and RBI, have been destroyed by the BJP-led NDA government.
"It is our responsibility to join hands together to save and protect this great nation and also save democracy and the Constitution," he told reporters after meeting Prime Minister H D Deve Gowda and his Karnataka counterpart H D Kumaraswamy.
Naidu hinted that the 1996 model of forming the government with Deve Gowda as prime minister with outside support of the Congress, could also be one of the experiments to form the government at the centre.
"PM candidate, we will decide. All of us will join together. First our aim is to protect democracy and save nation. What I am saying is, Congress is the main and major party. If you see only one experiment, that is under Deve Gowda's prime ministership. At that time, the Third Front had come to power.
"Then we had take support from Congress from outside. That is the only experiment... " he said.
Asked whether he is refering to 1996 model of forming government, he said, "I am interested in nation and consensus. Everybody will join together. There is no organisation as of today.
I have taken some initiative and I am meeting everybody. After that, we will meet and decide how to go about."
Naidu, who had last week met heads of several opposition parties, including Rahul Gandhi, had termed his party's alliance with the Congress a "democratic compulsion" to protect the country.
Naidu also said the prime ministerial candidate would be decided at later stage, but at this point of time, it was necessary for all opposition parties to unite together to not only save the country, "but also save institutions from destruction by central government."
"It is an initial exercise (of bringing opposition parties together). After that, we will work together," he said.
He said West Bengal Chief Minister Mamata Banerjee and Kumaraswamy were holding rallies in January.
"These things are happening. How to work out the unification of the parties and how to go about thereafter, we will all see in course of time," he said.
Lambasting the Centre, Naidu alleged that it was using CBI and income tax departments to "control opposition,"
conducting raids "indiscriminately and harassing politicians, which people have been witnessing even in Karnataka and Tamil Nadu."
Such raids also have been conducted recently in Telangana, Uttar Pradesh and Bihar besides Gujarat, he alleged.
"Even as these raids are taking place, Prime Minister Narendra Modi is not making any statements," Naidu said.
"Besides, destroying the insititutions, the Indian economy is in doldrums as demonetisation did not have good effect on it," he said.
Petrol prices have been increasing day by day and rupee is also depreciating, he added.
The TDP broke away from the BJP-led National Democratic Alliance in March this year.
The TDP leader had also held meetings with Nationalist Congress Party (NCP) chief Sharad Pawar, Jammu and Kashmir National Conference (NC) patron Faqooq Abdullah and Samajwadi Party's Mulayam Singh Yadav and Akhilesh Yadav during the visit to New Delhi last week.
In the past, the TDP has been a part of the NDA-I as well as NDA-II dispensations till it exited in March.